Add default value in config for default database to use for all queries

[jacurtis]

Most of the time you will not be switching databases often, so defining a default database to use would make this much easier to use.

Of course we still need to be able to override this value if a user is using the non-default database.

My thoughts are to create a setting in config/mongo.php which is default_database and that will be set on the __construct() method as the assumed database to use unless set.