Improve release packaging

Compress the tar package with gzip.  (Tarballs are almost never distributed uncompressed.)

Include the OpenSeadragon version in the names of the tar/zip files and in the name of the packaged top-level directory.  Also include `-bin` (e.g. `openseadragon-bin-0.9.124.zip`) to distinguish them from source packages.

Include the licenses and changelog in the packages.

Move the packages out of the website root into a `/releases` directory, and don't delete old releases when releasing a new version.  This should resolve @Decipher's concern in #72.