Add a "getCollectionNames" command

Right now drivers have to scan system.namespaces with a heuristic (disinclude any namespaces containing a $ character), and modify the output to strip off the database name portion of the namespace. For drivers which don't have a helper, users may be intimidated by what is currently required to get the list of collections, or may feel that their implementation is shaky or not future-proof.

Further, as far as I can tell it is not documented anywhere how to implement this correctly (short of reading the code for drivers which do implement such a helper).