grb.py: add a way to count our subscribers
grb.py: attempted bugfix to subscribers count

This example: https://github.com/yukkurisinai/yo-golang/blob/master/yo/yo.go
makes me think that maybe we should be issuing a POST, not a GET. I can't
test that out on Yo's API docs sample page since it hardcodes that, so let's
deploy and see if it works.
grb.py: after some curl experimentation, got this to work
grb.py: clean up configuration a bit

Centralize the config-getting stuff into a function, and use that
to remove some not-really-necessary temporary variables.
grb.py: collapse SetKey and SetEmail to a single SetConfig

Because that is the sensible thing.
grb.py: better comment at head of SetConfig class