Add support for reading modem/line status flags

[jgillick]

This PR adds bindings to the FT_GetModemStatus method as well as creates an event emitter that fires ever time any of the modem/line status values change.

I'm planning to use this in order to use the "input" serial wires -- like DCD -- as simple input indicators along-side serial communication.

Usage:

var device = new ftdi.FtdiDevice(0);
device.open({
  baudrate: 9600,
  databits: 8,
  stopbits: 1,
  parity: 'none'
}, (err) => {
  device.on('modemStatus', function (status) { 
    console.log(status);
  });
});