Add events handling functionality. on() function and EVENTID object.

Example:
```javascript
var Nxt = require("./nxt").Nxt;
var nxt = new Nxt("COM4");

nxt.on('getinputvalue', function(data) {
	if(data[3] == nxt.INPUT_PORT_1){
		var adc = (data[11] << 8) | data[10];
		console.log('Touch ADC: ' + adc);
	}
});

nxt.sp.on("open", function () {
	// Init
	console.log("Robot connected")
	nxt.play_tone(440, 500);
	
	// Set up hardware
	nxt.set_input_state(nxt.INPUT_PORT_1, nxt.SWITCH, nxt.BOOLEANMODE);
	console.log("Hardware set up");

	// Start interval
	setInterval(function(){
		nxt.get_input_values(nxt.INPUT_PORT_1);
	}, 500);
});
```