PPM Added

Phew, pretty major feature implemented! I've added PPM code, I have to admit I have yet to test it, but it's based on code I used earlier, so it should work...

Anyway, it's got some pretty neat features:
-variable amount of channels
-variable servo center point (1500us, 1520us, whatever you want)
-variable servo travel (500us, 600us, whatever)
-variable pulse length (440us, 500us, whatever)
-variable end of frame length (10ms, 5ms, whatever)
-interrupt driven using timer 1, thus extremely low consumption of CPU cycles
-all settings changeable at runtime (even the amount of channels)
-frame synchronization, will always transmit a full frame, even when you change settings

That should be all you need to build a PPM transmitter ;-) Check out the SVN repository!