Update ring to handle HEAD requests
Load plugins with FS.createPreloadedFile

This should be replaced with FS.createLazyFile when
https://github.com/kripken/emscripten/issues/2445 is fixed.
Some fixes for compiling against emscripten 1.20.0

I had to back out emscripten 1.20.0 but I'm keeping these
changes.