Allow passing arguments through to the SerialPort constructor

I've been using some hardware that doesn't run at 57600 baud, unfortunately the speed that firmata.js connects to the serial port at is fixed.

This pull request lets you specify a `serialport` object in the `firmata.Board` options which is then passed on to the `SerialPort` constructor so you can configure it how you wish.

I've also included unit tests for the same and updated the readme.