MongoCursor constructor and related changes

1. C++ wrapper class for MongocCursor
2. Moved all get_client/get_cursor methods to mongo_common so that all cpp files can access those methods by including mongo_common.h.
3. Finished the MongoCursor constructor (currently assuming query = {"test_field" => some char of length 1}).
4. Added a simple test for MongoCursor (currently will throw an exception if the cursor fails to be constructed).