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.
This would be more clear to do with `if / else if / else` instead of two different tests and `toLowerCase` three times.

```
var typeMode;
type = String(type).toLowerCase();
if (type === 'low') {
  typeMode = 0;
} else if (type === 'high') {
  typeMode = 1;
} else {
   throw new Error('SCT input pulse type not set correctly. Must be either "high" or "low"');
}
```
Also note that I'm throwing an actual exception instead of just logging an error and returning, so that users get a stack trace with the line number they have to fix.