Manipulate user objects exclusively via commands (MongoDB 2.6)

Previous to MongoDB 2.5.3 adding and removing users was accomplished by reading and writing <db>.system.users. Starting with MongoDB 2.5.3 user management happens through a set of database commands. For this ticket the important commands are usersInfo, createUser, updateUser, and dropUser.

The goal of this ticket is to keep the existing add_user and remove_user API for PyMongo, using the new commands when connected to a 2.5.3+ server, falling back to the existing code on older servers.