Updated helper for bootstrap glyphicon release 3

This is a tentative to update the gem with Bootstrap 3 (and try pull request as well) glyphicons. As always, no guarantee it works so you may want to double-check ;)

You should create a branch so in our gemfile we can have

`gem 'rails_bootstrap_helper', github: 'hoangnghiem/rails_bootstrap_helper', branch: 'bs3'`