Dropping Rails < 3.2 support

The Rails maintenance policy indicates that rails < 3.2 will not be supported after Rails 4 release (see: http://weblog.rubyonrails.org/2013/2/24/maintenance-policy-for-ruby-on-rails/).

I think it would be OK to drop Rails < 3.2 too. This way it would be possible to enjoy `ActiveSupport::Concern` and other neat stuff :)

Since it's a backward-compatibility-breaking change, it makes sense to release a major version (which we should have done when dropping Ruby 1.8, now that I think of it).

What do you think ?