DBPSK, DQPSK, refactoring

I’ve just merged in the changes for uniformly handling a wide variety
of modulations include GMSK, DBPSK and DQPSK.

The old gnuradio-examples/python/gmsk2 directory is now called
gnuradio-examples/python/digital to better reflect what’s in there.

There are three main examples:

benchmark_rx.py
benchmark_tx.py
tunnel.py

These all accept a -m parameter on the command line. The
currently supported values for are gmsk, dbpsk and dqpsk.

The old modulation specific examples such as benchmark_gmsk_{tx,rx}
are now gone.

Details here: http://gnuradio.org/trac/changeset/3662

Have fun!

Eric