Allow usage of Mongoid prior to version 3.0

[rudy-on-rails]

Hi.

Since on our project we're using Mongoid 2.4, we're having problems with current version gem's requirement of mongoid 3.0. I've noticed that Mongoid 2.0 was allowed on a prior version of the gem, so i have created a class to help dealing with different requirements amongst different versions of mongoid and delegated the logics of interaction with mongoid that along with the versions to that class.

I've also included a default .rvmrc file to load a preset env case dev is using rvm to control versioning of ruby and gems.