Convert basechurch-api into a Rails engine

 - [x] Create a mountable plugin called basechurch
 - [x] Move everything from app to plugin
 - [x] Remove the initial Rails app
 - [x] Create gemspec file to package as gem

This PR will fix #9.

Resources:

 - Rails Engines with Rspec: http://www.andrewhavens.com/posts/27/how-to-create-a-new-rails-engine-which-uses-rspec-and-factorygirl-for-testing/
 - Running migrations for rails engines: http://pivotallabs.com/leave-your-migrations-in-your-rails-engines/
 - Using Devise in a mountable engine: https://github.com/plataformatec/devise/wiki/How-To:-Use-devise-inside-a-mountable-engine