Add test for proper IssueEvent.issue

When the issue event JSON is obtained via the "List events for an issue"
API, it does not contain information about the parent issue.  In this
case the second argument to the IssueEvent constructor must be the
parent Issue, but Issue.iter_events() does not arrange for this.  As a
result IssueEvent.issue is bogus and __repr__() breaks.  Add a test for
this case.
Fix IssueEvent construction by Issue.iter_events()

IssueEvents returned by Issue.iter_events() had a bogus "issue"
attribute and thus a broken repr.
Fix up IssueEvent class initialization
Go back to the old way of doing things
Fix up tests
Merge branch 'pr/220' into develop

Conflicts:
	github3/issues/event.py