Initial version of Xbus support.

This is the initial version of support for the JR Propo/XBus MODE B protocol. 

Tested with these cli-settings (on a NAZE32 rev 4) using a serial cable from rx to flightcontroller:
set serialrx_type=5
feature SERIALRX

The firmware currently supports the MODE B version of the XBus protocol.
- Make sure to set your TX to use "MODE B" for XBUS in the TX menus!
 - See here for info on JR's XBus protocol: http://www.jrpropo.com/english/propo/XBus/

NOTE: This is actually multiple protocols that uses this standard. Known under the UDI, Jeti, Multiplex and SRXL protocols. Hence with more help/test multiple new brands of tx/rx systems can be supported. I know there are some small differences between these dialects of the protocol (sync byte/id etc) and hence I named it "xbus" as this is the one I have tested. 
More info can be found here: http://www.rcgroups.com/forums/showthread.php?t=2018101