#2 Description file improvements

[Robert Szewczyk]

Description files should contain the message size
expected by the application. The weird corner case is
where an application uses multiple description files,
each of which specifies a different packet size.

Description files should also contain extra information
about the requirements on the message subsystem: do we
expect a signal strength reading? Do we expect a CRC?
if not, these fields should not be included.