Fixing #2

Hi! 

I think I have a workaround for the problem in #2. The thing was that ``.locale_suffix`` was undefined in certain cases, related to Devise at least in my first failing test. After having a look at how the method is included in views and controllers, I refactored that into a module so I could try including it to more classes / modules if needed. 

I ended up calling ``.locale_suffix`` with the full name (``RailsTranslateRoutes.locale_suffix``) and now everything's working for me. I think the change makes sense, in any case.

I could not run the test suite in my machine (```<class:RailsTranslateRoutes>': uninitialized constant RailsTranslateRoutes::ActionController (NameError)``), so I could not test the solution apart that with my own project. 

What do you think?

Cheers!