New features

Hey Mateo,

I stumbled on your project yesterday trying to find a better way to integrate locales. Found it looked better than the other options out there plus you were already making use of http_accept_language!

I added a few things to it, let me know what you think. All tests are passing. It's quite a lot of changes in a single pull request, sorry about that.

Mainly the changes are:
- Add a current_locale method
- Automatically adds a before_filter for set_locale on ActionController (couldn't find a reason not to have it. Plus you get a localized version of other controllers that do not inherit from ApplicationController, like Devise would?)
- Bundled it in a Railtie

Upcoming:
- Generator for config file
- link_to_locale helper method to change locales

I edited your Gemfile and gemspec and removed all your versioning thing in there. Looked quite deprecated to me, sorry if it wasn't the case.