Allow the user to set a serial port in board options

I've run into a few issues where the serial port is not properly detected by Johnny-five and right now there is no easy way to set the correct port through the options hash.  I've been [doing something similar](https://github.com/theycallmeswift/BreakfastSerial/blob/master/BreakfastSerial/BreakfastSerial.py#L23) over in BreakfastSerial.

Ex. 
```javascript
var board = new Board({ port: "/dev/tty.usbfoo1234" });
```

Thoughts?