Seek

I implemented and expose the seek method.

I implement it only for http and rtmp. Rtsp return an error.

The only parameter is the second to seek from the beginning of the stream. 

We can only seek to keyframe. If we are not on a keyframe, it will take the closest one (as described in http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/net/NetStream.html#seek()).

Linked to #96