Ensure "watch" notices changes to any built-openseadragon file

Also remove obsolete pattern.
Move downloads to GitHub Releases system
Drop releases page

Link directly to the GitHub releases page, and convert our old releases
page into a redirect.
Move old release binaries out of built-openseadragon directory

since they will no longer be managed by the openseadragon/openseadragon
Gruntfile.
Add README documenting that releases directory is obsolete
Merge pull request #23 from bgilbert/releases

Move release list and artifacts to GitHub releases page