enable a video streams bitrate

expose the bit_rate of a video stream to the VideoStream class

Rational:
The value returned by Format.bit_rate is not the same as the video streams bit_rate. Exposing the video stream bit_rate is useful when setting the -b:v (bitrate video) parameter during ffmpeg encoding.


tested on Mac OS X 10.6 and 10.7 with ruby 1.9.3-p385