VO Cone Search -- 2 feature requests, SAMP doc prep

This includes:
* #933 - Expand Cone Search to search for all catalogs with one call.
* #945 - Better integrate ``astropy.coordinates`` with ``astropy.vo``.
* lpaioro/astropy#3
    * Move most the contents of vo/index.html to vo/conesearch.rst.
    * Modify vo/index.html to only include conesearch (which then calls client and validator) and eventually SAMP RST.

Hopefully will be merged in time for v0.3 (#568).