Unrequire matplotlib

Ran `pip install git+https://github.com/colettace/wnd-charm.git@unrequire_matplotlib` from within a freshly created and activated virtualenv and completed without error (installed numpy and scipy). `python -m unittest discover` in tests/pywndcharm directory came back with an unrelated error (which will be fixed by my tiling2 branch)

This PR also includes some new unittest code focusing on matplotlib functionality which turn on if `matplotlib` is there and turn off if it isn't.

Amended README.md to reflect that matplotlib is no longer required.