Fixed FlowControl (Actually tested this time ;)

Flowcontrol is working in this commit, I've actually tested it this time. (Oh, and thanks for the Selfsigned drivers, it  now works for me on Win 8)

The USARTClass.* files go into cores/digix

wifi.setFlowControl(true) turns it on. (This does NOT call the AT command, it only tells the USARTClass to start setting the CTS pin, I may implement RTS later even though a majority of the time it isn't needed.)