Add files to plays

* Upload photos or videos to plays
* Improve styling using Bourbon and SASS
* Improve test coverage with Shoulda-Matchers
Check the [documentation](http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-change_column_null), you may just be able to pass the boolean instead of the full hash, e.g. `change_column_null :media, :play_id, false`