Properly protected private methods in IconHelper
Moved IconHelper to default path for rails engines
Added an integration test to ensure icon helpers are loaded
Cleaned up icon helper tests
Changed method signature of fa_stacked_icon to accept :base option

New helper signature:

fa_stacked_icon "twitter", base: "check-empty"
Added support for passing other tag options to icons

e.g.
fa_icon "user", data: { id: @user.id }
# => <i class="icon-user" data-id="123"></i>