Add support for adding context to commit status.

This groups statuses from multiple contexts to return single combined
status. More information here:
https://developer.github.com/changes/2014-03-27-combined-status-api/
Preserve api compatablity with previous version.

Support adding commitstatus without context.
Merge pull request #97 from suryagaddipati/add_context_to_commit_status

Add support for adding context to commit status.