setup/version-legacy-packages - #174

+1 that's a good idea.

I have to say, this whole legacy shim apparatus is pretty awesome. I can't wait for the first Astropy release so we can start trying to use it.

One thing I might add to this: In addition to __version__ it might be nice if each package could provide a add_legacy_alias a whole dict of arbitrary variables to include in the generated __init__.py.

For example, PyFITS has an __svn_version__ variable. I don't know that there's any code that relies on it being there, but some users might check it. I could use this a place to record the svn revision each time I do a merge to astropy from pyfits.

I'm guessing I'll want to add asciitable as a legacy package.

This now includes @iguananaut's extension to include arbitrary values in the legacy shim's __init__.py.

Perhaps a short section should be added in the developer documentation to mention how this system works? I can actually imagine this mechanism might be useful for to people who might not be following the development too closely (including e.g. adding legacy shims for old packages to affiliated packages rather than the astropy core).

@mdboom - are you thinking will add the aforementioned documentation section in this pull request, or should I merge this and just add an issue about it to be dealt with later?

Sure, let's go ahead and merge. I had meant to add developer docs, but it's a little tricky -- the usage of this is already documented, so that's the basics covered, at least.

Ok, I'll try to look for a space to put this in the work I'm doing for #162, and afterwards you can fill it in whenever you find the time. Thanks!