Add Mongoid adapter.

Hi guys,

This PR contains adapter for [Mongoid](http://mongoid.org/en/mongoid/) + tests + small change like

+ added simplecov into local testing process (on Travis we are still using coveralls)
+ added "/spec" filter for simplecov

I've also changed rails version to 3.2.x because mongoid doesn't have support for rails 4.0.0
If you don't like it, I can try to make Gemfile with rails 3.x.x and Gemfile with rails 4.x.x for travis (your can check it [here](http://about.travis-ci.org/docs/user/languages/ruby/#Testing-against-multiple-versions-of-dependencies-(Ruby-on-Rails%2C-EventMachine%2C-etc))), then mongoid adapter will be disabled for rails 4.0.0

Please let me know if you have any suggestions