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)
switch backend only if necessary

This suppresses warnings on older matplotlib version, when the
backend is switched to on already active version.
Merge pull request #63 from aheld84/master

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