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
added and implemented _getSafeElemSize()
fixed _getSafeElemSize() calls
fixed _getSafeElemSize()
refactored _getSafeElemSize()
cleaned up docBlock and tabs in _getSafeElemWidth()
Merge pull request #165 from bgilbert/releases

Update for GitHub-hosted release binaries
Merge pull request #177 from pnorcross/_getSafeElemSize

(hopefully) Final fix for issue #109, Viewport Broken on Hide.
Changelog for #177; minor doc tweaks
Add a destroy function on the viewer to clean up and remove elements
created by open seadragon. Add removeAllHandlersForAllEvents to clean up all events on destroy. Clear
the onDraw callback on Overlay destroy.
feature: navigatorPosition, option was available but not completely implemented
fix: calling viewer.destroy() multiple times throw an error. And a simple destroy to mousetracker
updated chagelog to reflect navigatorPosition patch
fix: comment updates and add a simple test for viewer.destroy
update the comments and unit test
Working on rotating images.
So far only 90 degree rotation is supported.
Only the image is currently being rotated.
Overlays, debugger, and the navigator still need to be updated to support rotation.
Clicking on a button control no longer refreshes page