Rails 4 support

Hey guys!

Rails 3 + Ruby 2.0.0, 1.9.3, 1.8.7 - green;
Rails 4 + Ruby 2.0.0 - green;
Rails 4 + Ruby 1.9.3 + SQLite3 - one red because of weird problem with mb_chars;
Rails 4 + Ruby 1.8.7 - there's no support of 1.8.7 in Rails 4.

Compatibility module added to remove AR 4 deprecation warnings.

There's Edge version of Rails 4 in Gemfile because of ActiveRecord 'having' clause bug in 4.0.0.beta1. We could wait until next version of Rails released.