Intersphinx

Use Sphinx’s own intersphinx files and APIs to get all symbol names.  That’s both more robust and gives us a better introspection into the actual types of symbols.

Todos:
- [x] front page as reported in https://github.com/hynek/doc2dash/issues/27#issuecomment-50266807
- [x] bring coverage back to 100%

Should fix #27 and #25.