Rely on Sphinx' "any" role to make docstrings more legible

Sphinx >=1.3 has the "any" role (http://www.sphinx-doc.org/en/1.4.8/markup/inline.html#role-any) which allows one to write e.g.
```
`Axes`
```
instead of
```
:class:`Axes`
```
and will automatically figure out the proper markup role (and warn if there is an ambiguity).

This would make non-rendered docstrings quite more readable.