Enhanced playback rate

Added 2 API functions to the feature: 

1- setNextOrPrevSpeed – add the ability to set the video play faster or slower. This notification gets a delta argument (e.g. 1 to play the video faster than current played speed, -1 to play it slower than current played speed) 
2- setSpecificSpeed – sets playback speed to a specific speed by an argument. This API assumes that the new speed is in part of the optional speeds that were configured in the ‘speeds’ attribute.  

The test page now has 3 additional buttons - faster, slower and reset to show the new API abilities.
mwEmbed/modules/KalturaSupport/tests/PlaybackRate.qunit.html