Add protocol wrappers to the generic protocol

Add protocol wrappers to the generic protocol

This adds two wrappers to the generic protocol to help interfacing
small microcontrollers (arduino et. al.)
When receiving binary data over a sequential link, it's hard to tell where a packet
starts and where it ends. Two existing solutions gets implemented with this patch:
KISS (http://www.ka9q.net/papers/kiss.html) originally developed for ham radio
packet radio devices and STX/ETX byte stuffing.
This adds only the OUT direction, IN will be implemented over the next weeks.

For implementation details on the client (microcontroller's) side, check out
http://gitorious.org/flightgear-pmpt/cmpt/

This implementation is used in the c172fg project to be presented at FSweekend 2013.