Querying $cmd collection works now

Mysteriously (or not so mysteriously), querying the $cmd collection works after C++ team implemented limit,  skip, etc. I guess the driver was smart enough to figure out that if limit wasn't set to -1, then there was no way that it could be a command :-).  

1) Added a few simple tests (createCollection, dropCollection)
2) Refactored + added a few MongoCollection methods

More tests to come later now that command queries work!