Add initial USB serial support.

Using the teensy board usb_serial example code from pjrc.com. This has a
smaller footprint than LUFA for simple serial I/O (may still migrate to
the LUFA libraries at a later stage).
USB bugfixes for successful comms
Move buttons from (non-existant) PC0-PC3 to PF4-PF7.
Enable serial debug output on PD3
Servo control updates

Servo output signal (OC1A) moved to PB5. Timer1 values changed
to reflect the new CPU speed of 16MHz