Frontpage building tools

This is a somewhat hackish but nonetheless robust mechanism for re-building the frontpage.

The frontpage (http://www.astropy.org/astroquery/) is generated directly from the `README.rst` file.

To build the frontpage:
```
cd frontpage
make
```
This will build the `index.html` file in the `gh-pages` branch.  You then have to manually push that branch, but that should be easy.

This means that the `gh-pages` branch cannot be destroyed, as is often done with jekyll builds: the stylesheets in gh-pages can be directly edited and should be preserved.