Support OS X

[mcolyer]

There isn't an easy way for OS X users to create a fake serial devices and it's not automatic on Linux. To make it work on OS X and to improve the experience on Linux, we should port the serial communication to libftdi.

It would probably be best to refactor the serial port class so that it's injected into Hacklet::Dongle rather than created there. Once that is complete then implement a new class called Hacklet::SerialPort.

https://rubygems.org/gems/libftdi-ruby/
https://github.com/open-ephys/GUI/wiki/libFTDI

This will introduce a new dependency for OS X and linux:

OSX brew install libftdi
Ubuntu apt-get install libftdi-dev