Added totalPin mapping for the MEGA

Mapped The totalPins for the mega and fixed crash when `Board.Pins.translations[ type ]` returns undefined.

Used this to map pins, it will make pin mapping easy in future for other boards:

	five.Board().on("ready", function() {

	  var pi = [];
	  var pins = this.firmata.pins;

	  for ( var i in pins ){

		pi.push( pins[i].supportedModes.length );
	  }
	  console.log( pi );

	});