Don't publish release binaries into site-build repo

We're going to be hosting them in GitHub Releases instead.
Stop excluding site-build/built-openseadragon/releases from clean

The historical releases directory in site-build is no longer kept in
built-openseadragon.
Stop copying changelog.txt to the website

It's not referenced from the site itself, and the GitHub releases page is
nicer.
README: Update link to releases page
Merge pull request #165 from bgilbert/releases

Update for GitHub-hosted release binaries