Replaced nose.plugins.skip.SkipTest with unittest.SkipTest

To avoid dependency on nose (part of #594).

Topographica still depends on nose for tests, but that's ok (right?).

However, what about the ipython-related files in dataviews and param? I guess they are all optional, so it doesn't matter anyway. Just in case, I made a temporary branch of dataviews so you can see what I mean: https://github.com/ioam/dataviews/compare/skiptest_from_unittest. I'll delete that branch once someone lets me know what to do.