Added playback rate control

used playback rate control from msdn example, currently reverse playback
seems to not be working but forward rate changes (rates >=0) are smooth. Perhaps needs to check against minimum and maximum playback rates? ofxwmfvideoplayer does push an error for negative playback perhaps it is not supported?

for set rates properties
http://msdn.microsoft.com/en-us/library/windows/desktop/ms696979(v=vs.85).aspx

supported rates
http://msdn.microsoft.com/en-us/library/windows/desktop/ms699861(v=vs.85).aspx