Thanks! This change makes perfect sense to me, however there're some things that need to be addressed before this could be merged. I left some comments below.

Also, could you please add some tests? 3 tests should be enough, I guess:

1. g.geocode() being called without any args should raise a ValueError
2. g.geocode('london', place_id=42) should raise a ValueError (since 2 incompatible args are being specified)
3. g.geocode(place_id=*some_valid_palce_id*) should return a valid Location, just like a plain g.geocode('some city') call.

Thank you for the edits! Just a couple more and I think it would be ready.

Merging, thanks!