Add microseconds mode

[zackees]

Please make it possible to do microsleeps. I would like to use this to microsleep 100uS intervals

timer.setMode(kModeMicroseconds);
timer.setTimer(5000); // 5 milliseconds

[duff2013]

The clocks that I set up are for milliseconds and can't do micro's and not all the Teensy's have the same clocks to use. It might be possible but not something I'm going to support in the near future but I'll keep this issue open to look at it at least.