Compress package tarballs with gzip

Tar files are conventionally compressed, often with gzip.  xz would be
better but is not as universally supported; in particular,
grunt-contrib-compress doesn't support it.
Use maximum compression when creating packages

grunt-contrib-compress defaults to minimum compression.
Rename {tar,zip}ball and packaged directory

{Tar,zip}balls now have the form openseadragon-bin-0.9.125.{tar.gz,zip},
containing a directory openseadragon-bin-0.9.125.

Including the project's version number makes it easy to determine the
version of a random release sitting around a filesystem, and ensures
that unpacking a tarball won't overwrite another unpacked tarball or
write into a checked-out repo.

Adding -bin distinguishes the packages from source tarballs, which are
conventionally <project>-<version>.{tar.gz,zip}.
Include changelog and license files in packages

A README would be good too, but right now README.md consists mostly of
things that are only meaningful in a Git checkout.
Don't delete old releases from the website

It's convenient for old releases to be available indefinitely so that
deployment scripts can retrieve a particular qualified version.  This
won't affect the disk space requirements of the repository, since the
old packages will be in the Git history anyhow.

Move packages to a subdirectory of the site root to avoid causing
permanent clutter.
Drop obsolete .gitignore entries
Drop unnecessary quotes
Stable Builds wording improvement
Merge pull request #75 from bgilbert/dist

Improve release packaging