BSON 2.0.0 (DNMY)

This removes Moped's own BSON implementation, with the new 10gen BSON gem 2.0.0, which is currently residing in the meta-driver here:

https://github.com/mongodb/mongo-meta-driver/tree/master/ruby

This pull is a WIP, but currently all tests are passing. We need to do the official BSON gem 2.0.0 release before bringing this into master.