A few miscellanous tweaks to the test runs:

 * Fix an issue where an error could occur when running setup.py test
   with the -P option but there are no Sphinx docs for the selected
   package
 * It turns out running pytest_ignore_collect causes paths to be
   ignore for all of pytest, not just a single plugin.  Kind of a pain,
   that.  This tweaks how DoctestPlus uses pytest_ignore_collect to
   implement the doctest_norecursedirs option.

It should also be pointed out that prior to these fixes the tests for the `astropy.sphinx.ext` package were not being run due to the `doctest_norecursedirs` issue.  Now that they run it turns out that they're failing with Sphinx 1.2.2.  That should be looked into as a separate issue.

I came across this problem in the first place in the context of trying to transfer `astropy.sphinx.ext` to live in `astropy_helpers` per the discussion in astropy/astropy-APEs#5