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.
Shouldn't the third element in this array be 'Pulse In`? When I was reading too quickly, I was getting errors that:
```
Error: SCT is in use by _
```