Update Rails to 5.1.0, convert to API-only application, style refactorings

[ahcarpenter]

• Update to Rails 5.1
• handle CORS
• add Rubocop, refactor style to more closely align with community style guide
• convert to API-only app

Resolves #5, #7, #8, #9

[ahcarpenter]

@apai4 any thoughts?

[apai4]

@ahcarpenter Thanks a bunch for doing this, this looks great!

I'd like to wait until acts-as-taggable-on and devise release official gems for Rails 5.1 instead of relying on GitHub for those dependencies. I'll pull this into a new branch where we can continue to accept changes for 5.1. Once the new gem versions are released we can update master to 5.1

[ahcarpenter]

@apai4 sure thing!

sounds good!