Fix Sphinx docs links

I wanted to help out a bit by turning on the `nitpicky` mode for the Sphinx build and fixing the astroquery docs links:
https://gist.github.com/cdeil/10836768

But I immediately ran into a weird issue that I don't know how to resolve:
https://gist.github.com/cdeil/10837380

For some reason `import lxml.html` doesn't work when run in `eso` via `python setup.py build_sphinx`, even though `import lxml` does work there and `import lxml.html` works for my installation when run directly.

I saw that `lxml` is being replaced by `bs4`, but if someone knows what the issue is here and how I can work around it I could finish this PR now.