Made upsert return reference to created document

When upsert is true and it can't find an existing record to match the query, it calls self._insert which will return a ref to the newly created doc as the 2nd param to the callback, but the upsert logic wasn't catching a 2nd param (only the first param: err) so I just added the 2nd param as 'newDoc' and passed it to the upsert callback instead of 'true'.
Updated upsert tests to check if newDoc has been sent back successfully, modeled after other insert tests
Updated README to reflect slight upsert change
Merge conflict resolved
Added a test on modification of upserted document