Variable Speed Support added

Hi Bob,

I've added the client code for the "Presto" variable speed sound library, so that Car Cast can now playback podcasts at variable speeds using both ["Presto"] (https://play.google.com/store/apps/details?id=com.aocate.presto), which is kinda buggy, and my own ["Stable Speed"](https://play.google.com/store/apps/details?id=com.stuntbyte.stablespeed) 

Cheers
Kerry