Issue#53 - Changes to allow 115200 baud rate to be set for USB link.

As discussed here are some siple changes to add a preference for setting 57600 or 112500. (defaults to 57600)
After setting is changed it will be used when next Connect is done.
Tested on Aus Slider and it works OK.
Baud of 115200 allows connection to APM (V1 ONLY) with USB cable (instead of 3DR radio / wireless type connection)