Adds github triggering from status updates

This adds support for triggering on github status updates.

Config schema for the github trigger has been updated to accept a list
of statuses, in the "github_user:context:status" format.

Change-Id: I15aef35716ddbcd1e66f84a73d27ca2689c936e4
Co-Authored-By: Jesse Keating <omgjlk@us.ibm.com>
Signed-off-by: Adam Gandelman <adamg@ubuntu.com>
7 files changed