Added tests for generator method

* Updates test file names to use the `*_test.rb` standard
* Added tests for generator method
* Added dummy Rails app to facilitate generator test
* Added class to lib/mozart/rails/assets.rb for download method
* Updated references to asset constants to use Mozart::Rails::Assets
* Added vendor JavaScript files to vendor directory for easier generator use
* Updated gem dependencies
* Removed unrequired extra Rake TestTask
* Trying to get Minitest::Rails working with the Rails app - no go for 1.8.7/3.0.x :(
* Bumps gem version