MongoDB storage driver

This supersedes #94 (cc: @iampersistent). 
Does this function make sense for the Doctrine DBAL and MongoDB drivers? For both, it simply inserts a one-field record into a separate table/collection. None of the message methods seem to require us to create a queue. If anything, we only need this record to exist for listing queues, since we'd have no way to know about queues with no messages otherwise.