remove dependence on nose and WIP decorator

[rjleveque]

@wip does not seem to be used anywhere, unless perhaps in pyclaw (I only checked the Fortran repos).

At any rate I think this functionality can be handled more directly in pytest. After importing pytest, you can add a decorator

@pytest.mark.xfail(reason='WIP')  # for 'expected to fail'

or

@pytest.mark.skip(reason='WIP')   # to skip entirely

to any test.

So I've removed the nose dependency to address the issue in clawpack/classic#90.