Add submodule.

The Xbee library is in a separate git repo.
Fix Makefile.

Updated to work with xbee1.c and xbee1.h in the
include directory.
Fix Makefile.

Fix clean targe in Makefile.
Clean up comments.
Update .gitignore files.
Update .gitignore file.
Update Makefile.
Clean up comments.
Fixed compiler warning.