ActiveRecord 4.2 Support

Hi this is a pull request that's definitely not ready to go because it breaks support for ActiveRecord 4.0 and 4.1 entirely. It does add support for the beta versions of ActiveRecord 4.2 and Arel 6.0. Arel changed quite a bit internally for the most recent release, and I'm not sure how to do support for the older version. 

It also don't quite confirm to your guidelines, but I wanted to start discussion about how to include backwards support. I'll update to make sure it follows them shortly.