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 was thinking the explicit reference to ``k`` is good because that's what py.test calls it, so if you want to look up the syntax (which can be pretty convoluted if you want it to be), you need to know what to look up in the py.test docs.  So I was trying to be minimal to redirect interested parties to the docs for ``-k`` instead of putting much detail here. 

But none of that is a strong opinion - as long as the help clearly says something like "look up more details by looking up the py.test -k option", then I agree the name can be pretty much anything.  Do you have a suggestion?  Maybe ``-s`` and ``--select-test`` or something like that?