Install config file.

The installation of the config file used to be handled inside of `get_package_info` in these lines:

```
    # Add the package's .cfg file
    package_data[''] = [srcdir + ".cfg"]
```

which appear to have been removed in `astropy-helpers`.  I'd like to just put it back, but the name of the main source package is no longer passed to `get_package_info`.  So instead, I'm just handling it on an astropy-specific basis by adding a `setup_package.py` file to the top-level.

Affiliated packages have the same problem at present, whether they are using the 0.3 generated config file or the 0.4 explicit config file.