Support for mongoid 3.1

It would be great if this gem supported mongoid 3

[maca]

+1

[Aethelflaed]

and mongoid 4...

[goncalossilva]

I'm not using mongoid at the moment, so it's been hard to find some time to look into this.

I'll gladly accept pull requests, though!

[Aethelflaed]

Okay, I'm browsing the forks and found several ones which may match my needs.