RUBY-686 uri parser support for auth source and auth mechanism
RUBY-686 light mongo client cleanup, changing default db

Most other drivers all default to the admin db and they use this for the default on auth source. We are the odd standout here, so I'm correcting behavior to bring us in line with everyone else.

In addition, I cleaned up a few minor things (mostly) in this code.
RUBY-686 fixing tests for parser, client and connection to add support for source and mechanism