Test doctests in .rst documentation

This is a work-in-progress.  I'm not still entirely with happy with the details of how this is being done.

Importantly, however, the process of automatically running the doctests in the .rst files has found a number of places where behavior has changed, so I thought I'd share this early.  It would be nice to fix some of the documentation issues for 0.3 (as I think having working examples is a very important bit of polish), even if we can't agree exactly on how to automate doctest testing just yet.  I've put each packages' changes in a separate commit so we can cherry-pick them one-by-one -- I think everyone should review the packages they are most familiar with rather than reviewing the entire thing here. Feel free to pull individual changes into other PRs and commit independently of this one -- It's more important to fix broken examples in 0.3 than it is to keep this PR easy-to-merge, IMHO.

There are changes here that I have marked "TODO" because something is broken and I don't know how to fix it.  I will go through and create separate issues for each of these with a 0.3 milestone.

EDIT:  I should add, I punted on `io.fits` for now (too large and unwieldy) and on `vo` because the examples are all so "networky" they'll require some real mocking up to actually work in automated testing.