Fixes up MongoMapper null dsl.

Adds the "normal" database method if I should want to fallback.

Stores keys as strings and not symbols to be like MongoMapper.
Fixes bugs in Author model.

Author.create! uses first instead of find in lockfree create shim.
Author.create! now sanitizes input.
Author.create! can now take no arguments
Author.create now points to Author.create!
Author.discover! will now short circuit if Identity already exists.
Author.sanitize_params will now delete keys that should be hashes but
aren't.
Adds Identity.find_by_identifier.

This function will look up an Identity by their unique identifier, such
as their webfinger identity or email.

To make the checks better, all usernames and domains for Identity
purposes are lowercased.
Adds Author model tests.