MCP23017 PortExpander driving 4x4 KeyPad w/ interrupt / setWatch for 'keyDown' detection

@Wilberforce, emit along the the line of pseudo code below:

Setup of portexpander and enable interrupt on a pin A3:

var x = require("PX23017").connect(...);
x.enableInterruptOn("A3", function(state, time, lastTime) { // service interrupt on pin A3
    //   code to serve interrupt
  ),{ ...  }); // interrupt options...

with Internals of .enableInterruptOn(pinName, callback, optionsObj) {...

...
// handle the options object
...
this.on(pinName,callback); // prep for the emit(...
... 

and with emit(... on event

...
// figuring out event details, such as pin, state, time, lastTime
var pinNane = ....; // for example "A3"
...
this.emit(pinName, state, time, lastTime);