Make the distinction between source and platform releases clearer

The current wording seems to make the difference too subtle for a casual
reader; try to clear this up by:

- naming the latest release "source" instead of "stable" release, to
  make clearer what it is that we're showing the version for.

- reword the button to "Downloads" for a platform, which makes it a noun
  for the available downloads, rather than a verb which can too easily
  be taken to mean "download this version"

This should reduce the incidence of people complaining that the version
in the front is erroneous.
Merge pull request #364 from carlosmn/frontpage-version

Make the distinction between source and platform releases clearer