Update URLs to the new persona.org ones

This should eliminate a few unnecessary redirects and speed things
up.
Avoid blocking page rendering when loading external JS files

To enable browsers to continue to rendering the page without
blocking when they encounter these external Javascript files, it's
better to move scripts to the bottom of the page.
Merge remote-tracking branch 'fmarier/new_urls'
Merge remote-tracking branch 'fmarier/faster_rendering'
Update README mentions of "BrowserID" to "Persona"