use apple's codesigned python

In order to use notification center in python you must use Apple's code signed
version of python. See here:
<https://github.com/dbader/alfred-countdown-timer/pull/1>
and
<http://objcolumnist.com/2012/07/25/os-x-mountain-lion-api-highlights/>

This switches the wokflow and extension to use /usr/bin/python instead of the
environment's python.