expose pulsein to digital pins

[xmeow]

This should resolve #26, make hcsr04 works with newbit. Tested with the following code snippet:

def sonar(p):
  p.write_digital(0)
  sleep_us(2)
  p.write_digital(1)
  sleep_us(10)
  p.write_digital(0)
  p.read_digital()
  duration = p.pulsein(2000)
  if duration<0:
    return -1
  return (duration/2)/29.1

[dpgeorge]

Thanks for the contribution!

machine.time_pulse_us() should now be fixed by b7a76e4

The change in this PR adds a new method to pins, which changes (expands) the existing API so is probably not the best way to do it (because then it becomes hard to write code that works on V1 and V2).