Possible fix for #31

This is a possible fix for #31 (it also includes #30 for now). The basic problem is that we should be able to do:

```
python setup.py egg_info
```

in an affiliated package, and have it work if Astropy is not installed. With the current attached solution, things are still not quite right because in the egg-info directory, the SOURCES.txt file doesn't contain the full list of files, and the PKG-INFO file doesn't have quite the right version (the git string is not present). Either we decide to not care about this, or it seems we are going to have to include a copy of ``setup_helpers`` and ``version_helpers`` in affiliated packages (unless anyone can think of a better solution).

There's a third option, which is to not do quite so much for affiliated packages from the core package. For several simple packages I've been writing, I don't *really* need the sophisticated functions in Astropy to give me the list of sub-packages, data, etc. so we could trim down on all the 'magic' stuff and only really use Astropy to import the custom commands (``test``, ``build_sphinx``, the custom ``build`` and ``install``, etc.). But for package discovery, we leave that up to the developer, and they shouldn't use anything from Astropy core.