We should make the manifest consistent with OWA - https://developer.mozilla.org/en/Apps/Manifest#path-handling and specifically, "All fields that hold paths in the manifest must be absolute paths (for example, '/images/myicon.png'), and are served from the same origin as the app."
So in practice this means:
* ALL paths in the manifest must be absolute and relative to the origin.
* If we really need URLPrefix, it should follow the same rules. However, ideally this would actually be optional
cool! pushed to git and hg. URLPrefix is gone, replaced with the ability to define origin in a builtin manifest.