Make rails assets overrideable

[Eyüp Atiş]

Right now, we can not override these three assets because we are requiring them like this:
https://github.com/werein/x-editable-rails/tree/master/vendor/assets/javascripts/editable/rails

#= require ./rails/editable_form
#= require ./rails/data_classes
#= require ./rails/error_handling

[Jiri Kolarik]

Sounds legit, merging