Add test if quantities work with matplotlib

This adds a test if quantities work with matplotlib (see issue #1881).

On my Mac this leads to `python setup.py test -P units` simply aborting without giving an error message.
So I presume this is not the right way to add this test?

cc @mdboom @astrofrog
Perhaps I'm just being pedantic here, but these inline imports are no longer necessary.  You can just use `u.Quantity`, and matplotlib is already imported above (could add from matplotlib import pyplot maybe).