Pluggable caches and Python 3.5 support

• Dropped support for Django prior to 1.6
• Dropped support for Celery prior to 3.1
• Dropped support for Python 2.6
• Dropped support for Python < 3.5 - thanks @midnightlynx for 3.4/3.5 support
• Support for pluggable caches - thanks @rhunwicks
• Fixed setup.py license to match MIT license in repo - thanks @sodre

Support for psutil >=3.0

0.3.1

• Switched to the psutil 3.0 API.

Version 0.2.2: `update_frequency` now works!

Changelog

0.2.2

• The update_frequency argument to update_progress() wasn't properly
  delaying updates to the back end after the first increment. This resulted
  in many more progress updates to your backend, which means you should
  see performance gains with this release if you took advantage of that option.
  Thanks to Ilya Baryshev for the fix.
• Logging calls related to update_progress() are now at the DEBUG level
  instead of LOG. Hopefully your logs are much more readable, as a result.
  Thanks to Kyle Gibson for convincing me that things used for debugging
  should probably be at the debug level.