Samp sphinx fixes

@lpaioro In this PR I have rebased against astropy master and done a few fixes to the Sphinx documentation build.

I've also split up the `astropy.vo.samp` documentation into a few files I plan to write.
I think we can discuss on your PR what documentation exactly we want, but I think there should be some cool and common examples how to interoperate from Python with GUI applications like ds9, Aladin and TOPCAT to get people's attention.