Post VideoStreamReceivedKeyFrame notification when a keyframe is decoded

Ignore-this: 78c5f97590d4c484c3f4a0f7ade930dd

darcs-hash:20141021101619-56b4d-522e68996e289e1f0935acc0a185bde52d0809f4
Added __str__ methods for completeness

Ignore-this: b6d6bf8345be143d9b3db0ad177f794

darcs-hash:20141022204025-b22ec-249184589756d2290587b87270f1d2f1545b668b
Reset frame handler when closing FrameBufferVideoRenderer

Ignore-this: 636929760099703d7bb10dcb188d48fe

darcs-hash:20141023132116-56b4d-e23a0e5dd78ab487fea2d3f671f318cbf67f3672
Reset event_handler when closing RemoteVideoStream

Ignore-this: 23be48a71287667b7a501dc6b896af2e

darcs-hash:20141023132208-56b4d-7fbea33e08f44c0a1ddf2b19a1774ead2f56850f