Added V4 HMAC support

Appraisals

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+appraise "3.2" do
+  gem "rails", "~> 3.2.0"
+end
+
+appraise "4.1" do
+  gem "rails", "~> 4.1.0"
+end
+
+appraise "4.2" do
+  gem "rails", "~> 4.2.0"
+end

CONTRIBUTING.md

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+Contributing
+============
+
+We love pull requests. Here's a quick guide:
+
+1. Fork the repo.
+
+2. Run the tests. We only take pull requests with passing tests, and it's great
+to know that you have a clean slate: `bundle && bundle exec rake`
+
+3. Add a test for your change. Only refactoring and documentation changes
+require no new tests. If you are adding functionality or fixing a bug, we need
+a test!
+
+4. Make the test pass.
+
+5. Push to your fork and submit a pull request.
+
+At this point you're waiting on us. We like to at least comment on, if not
+accept, pull requests within seven business days (most of the work on Paperclip
+gets done on Fridays). We may suggest some changes or improvements or
+alternatives.
+
+Some things that will increase the chance that your pull request is accepted,
+taken straight from the Ruby on Rails guide:
+
+* Use Rails idioms and helpers
+* Include tests that fail without your code, and pass with it
+* Update the documentation, the surrounding one, examples elsewhere, guides,
+  whatever is affected by your contribution
+
+Running Tests
+-------------
+
+Paperclip uses [Appraisal](https://github.com/thoughtbot/appraisal) to aid
+testing against multiple version of Ruby on Rails. This helps us to make sure
+that Paperclip performs correctly with them.
+
+Paperclip also uses [RSpec](http://rspec.info) for its unit tests. If you submit
+tests that are not written for Cucumber or RSpec without a very good reason, you
+will be asked to rewrite them before we'll accept.
+
+### Bootstrapping your test suite:
+
+    bundle install
+    bundle exec appraisal install
+
+This will install all the required gems that requires to test against each
+version of Rails, which defined in `gemfiles/*.gemfile`.
+
+### To run a full test suite:
+
+    bundle exec appraisal rake
+
+This will run RSpec and Cucumber against all version of Rails
+
+### To run single Test::Unit or Cucumber test
+
+You need to specify a `BUNDLE_GEMFILE` pointing to the gemfile before running
+the normal test command:
+
+    BUNDLE_GEMFILE=gemfiles/4.1.gemfile rspec spec/paperclip/attachment_spec.rb
+    BUNDLE_GEMFILE=gemfiles/4.1.gemfile cucumber features/basic_integration.feature
+
+Syntax
+------
+
+* Two spaces, no tabs.
+* No trailing whitespace. Blank lines should not have any space.
+* Prefer &&/|| over and/or.
+* MyClass.my_method(my_arg) not my_method( my_arg ) or my_method my_arg.
+* a = b and not a=b.
+* Follow the conventions you see used in the source already.
+
+And in case we didn't emphasize it enough: we love tests!

Gemfile

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+source "https://rubygems.org"
+
+gemspec
+
+gem 'sqlite3', '~> 1.3.8', :platforms => :ruby
+
+gem 'jruby-openssl', :platforms => :jruby
+gem 'activerecord-jdbcsqlite3-adapter', :platforms => :jruby
+
+gem 'rubysl', :platforms => :rbx
+gem 'racc', :platforms => :rbx
+
+gem 'pry'
+
+# Hinting at development dependencies
+# Prevents bundler from taking a long-time to resolve
+group :development, :test do
+  gem 'mime-types', '~> 1.16'
+  gem 'builder'
+  gem 'rubocop', require: false
+end

LICENSE

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+
+LICENSE
+
+The MIT License
+
+Copyright (c) 2008-2015 Jon Yurek and thoughtbot, inc.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.