fixes #62 (visualization is broken with latest macports dependencies)

adds package `mplbackend` providing a function `ensure_mpl_backend`
to choose a matplotlib backend.

The backend is now set explicitely in
* the gui entry script (wxagg)
* the test-all.py script (agg)
* the setup.py test run of pyphant and OSC (agg)
* the OSC/OSC/data.py example (wxagg)
* all test modules involving plotting (agg)