New library for PWM playback from SD cards: SimpleSDAudio

@Tuttut: I saw my changes in the newest version, I will have to try it out in 0023. Thank you for abstracting the SD card library so that I only had to change the "0" level. That made it much easier.

I have a video of a basic 8-bit 78.125kHz stereo sample running on my ChipKit Uno32 using a minimal setup. Check it out:

-Thomas