Add support for mongodb 2.4, and implicit authentication

Hi, updated pull request including the fixes you asked for.

The pipe/colon thing: I'm trying to represent this sort of JSON structure:
{'database': ['role1, 'role2', 'role3'],
 'database2': ['role1', 'role2']
}

so the above would be: database:role1,role2,role3|database2:role1,role2

I'm happy to change the representation though, just let me know.



I had to add OperationFailure to the outer exception handler as my version of pymongo (2.5.2) throws that if authentication fails.