Add sessions to Events and event payload members

By making Events subclass GitHubCore, it will be passed a session when
created. When that happens, any element of the payload that we convert
to a github3 object will also have the same session.

Fixes #303

(cherry picked from commit f6510976d0635ebce001a462a5bb530b5cfed763)

Conflicts:
	github3/events.py
Fix old events tests

(cherry picked from commit 7a60063fc52816fee32663ed1a918f756a6520e0)