Add basic Sphinx docs

Add a Sphinx skeleton with a basic project summary and an Installation section.  Also, update the README to refer to the Sphinx docs and delete the existing INSTALL.rst.  This is what IPython does, and it avoids having two copies of the installation instructions.

Building the docs requires the `sphinxcontrib.napoleon` extension to be installed (handles numpydoc-style docstrings).

I didn't know what to fill in for `authors`, so this should probably be edited.

I've also turned on hooks for readthedocs, so our docs will be automatically built and hosted at http://distarray.readthedocs.org.

I also added a few lines to `.travis.yml` to make TravisCI try to build the docs.

It's a start for #93