Allow to select MongoDB API version (eventual target 0.9 support)

[rened]

I would like to discuss how we could start supporting different versions of MongoDB. I think it would be great to be able to select between 0.8 / 0.9. As both versions might be around for quite a while I don't think it is feasible to just switch from 0.8 to 0.9 a certain point.

My main priority here is make the code changes required to support different versions - the actual support for 0.9 is a different issue.