modified chat_sanitizer to add an access code to the transmitted packet.

my_qpsk_demod no longer has differential setting; removed from grc.
Merge branch 'master' of github.com:gnuradio/gr-tutorial
Merge branch 'master' of github.com:gnuradio/gr-tutorial
tut7: updates

 - switched to constellation modulator / objects
 - fixed map issue for differential decoding
 - updated GRC screenshots