Dynamic 300 baud

[tahpee]

This builds upon the previous pull request to add MIC-E support.

This pull request adds dynamically selectable 1200 or 300 baud TX. The RX should still work at 1200 but is untested.

This might seem a little crazy at first glance but here in Australia we run a national VHF APRS network on 145.175MHz at 1200 baud and a national HF network on 10.147MHz at 300 baud. The VHF coverage is mostly good for the populated areas but if you are travelling in the outback outside the VHF coverage HF is a must.

Using a radio that works on both HF and VHF, a single modem can operate on both frequencies and baud rates. The radio can be controlled via another software serial port on the Arduino to send commands to put the radio on the correct band/frequency/mode before sending the position report.

It's probably a good idea to check the changes I've done to AFSK.* because I've mostly hacked it using educated guesses until I got something that worked.