Add Staff Graded Assignments XBlock to edx-platform

This adds the PyPi version of SGA (https://github.com/mitodl/edx-sga) to requirements and optional apps. I wasn't sure whether to add it to the github requirements or here.  It seemed cleaner to use the PyPi version.

@pdpinch @ichuang @nedbat @cpennington
For the moment, we want 3rd party XBlocks to be referenced using specific commit hashes from a trusted source (such as github).