Add support for MongoDB authentication

This commit adds support for user mappings to mongo users.  This requires that you specify `use_auth 'true'` in the server options when using authentication.

The reason for adding the new `use_auth` option is that PG's GetUserMapping function throws an error if none exists.  I was unable to find a way to check if a user mapping existed without throwing an error that aborts the query.  So we only call GetUserMapping if the server has `use_auth 'true'`.

Look it over and let me know what you think!