[WIP] Add 0.3.0-prerelease downloads to the page

Now that our status dashboard is green, we're almost ready to offer nightlies again.  We've just got a few windows issues to make sure aren't show stoppers, and then we can get people living on the bleeding edge again.

I would prefer to wait until windows builds have been OK'ed by someone who knows what they're doing (@vtjnash or @loladiro come to mind) before this is merged, but I won't cry if we offer broken nightlies on our download page.  It's happened before, and it will happen again.