Fix links to other classes in sphinx-generated html docs

Currently the sphinx-generated astropy html docs contain many cases where the html links to other classes don't work. Here are some examples:
* [NDData](http://docs.astropy.org/en/v0.3/api/astropy.nddata.nddata.NDData.html)
* [Angle](http://docs.astropy.org/en/v0.3/api/astropy.coordinates.angles.Angle.html)
* [Time](http://docs.astropy.org/en/v0.3/api/astropy.time.core.Time.html)

I've fixed these examples in the attached commit 48d32b4, the most common problem seems to be that developers put `~astropy.sub_package.Class` instead of `~astropy.sub_package.implementation_module.Class`.

So the question becomes ... are working links in the HTML docs important enough to warrant showing the "implementation detail" in which `implementation_module` the class is implemented in the docstring (which is what the user sees on the console).

The ideal solution would be to configure Sphinx to make `~astropy.sub_package.Class` work for the HTML build. Possible?