rbwebhooks: Add support for some additional events
Namely the events newly supported are review_request_closed,

review_request_reopened and review_request_approved.
The POST requests also contain additional header now that contains the event

that the request represents. The header is called X-ReviewBoard-Event.

I tested this by reading the server logs. It was failing for me just because the server was not reachable. Otherwise it looked ok, it should not throw any RB server errors...