Mongoid 3 version

Hi!
Here is result of my trying to make branch mongoid-3 working with latest mongoid gem.
I supposed to add a few features like Money, Integer, Float data types.
Also i fix some issues with generating paths for models with custom fields applied. 
Because of mongoid gem required ruby version >= 1.9.3 I've assumed to change requiring mechanism too.
Specs are passed all green.
Sorry for so big PR, but I just was intended to have a fully worked version and it is such now.
Thank you for your great gem!