Support for MongoDB 2.x

MongoDB 2.x returns nil instead of raising Mongo::OperationFailure when you hit a new collection, so I tried to add a fix in the least disturbing way possible.  All the tests pass now against v2. 

I also started messing with adding support for Mongoid 3 but had to back off because it will involve some refactoring. I left in some comments in spots I thought might be helpful.