Pin.readPulse() functionality

Basically does exactly what the title says. It implements readPulse functionality.

Check out `test/test_read_pulse.js` and mess around with the test to see if it's all good.

Comment/critique to your hearts desire.
Yeah, that makes sense. Could be tied in with the flag I described to check if the SCT is in use in a previous comment. Rather than a boolean, it could be an `enum { SCT_INACTIVE, SCT_PWM, SCT_READPULSE, SCT_NEOPIXEL }` and be used to dispatch the IRQ to the appropriate function.