Fix Mongoid 4 style OIDs coming through the API.

Mongoid 4 changed the way it serialized BSON::ObjectIds from a simple
string to something like {"$oid"=>"543e98556275734c1a300100"}.
This switches it back to using a simple string in the API.
Merge pull request #3 from biola/fix_mongoid_4_oids

Fix Mongoid 4 style OIDs coming through the API