Fix IssueEvent construction by Issue.iter_events()

When the issue event JSON is obtained via the [List events for an issue](https://developer.github.com/v3/issues/events/#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()` was not arranging for this.  As a result the `IssueEvent` had a bogus `issue` attribute and its `__repr__()` was raising an `AttributeError`.