Add libftdi support.

This is some initial ```libftdi``` support. Tested on MacOS 10.8.4 and Linux. On MacOS X (if installed) the FTDI VCP kernel extension must be unloaded, i.e. ```sudo kextunload -b com.FTDI.driver.FTDIUSBSerialDriver```. On Linux ```libftdi``` plays nicely with the ```ftdi_sio``` kernel module. I also updated the README to markdown and added build instructions.