Added kselect option to test runner

I got tired of constantly doing ``python setup.py test --args="-k <something>"`` to select particular tests by name, so this adds a command to the astropy test runner that does this directly (without needing to go through ``args``)
I would actually vote for sticking with the py.test options otherwise it's going to get confusing. ``-s`` is already used for something else in pytest (show stdout) and if we decided to make that available at the top level, we could no longer use ``-s``. We've been consistent so far, so I don't see a reason to change.