Fix infinite redirects

Fixes issues with poorly build websites that resulted in infinite redirect loops (#12)

Support automatic updates

As per Safari Extensions gallery requirements, the extension will now check for updates automatically

Cleaner AMP checks

Using @paulirish's updated observer disconnection code. Any potential performance issues from the extension should now be mitigated.

Load AMP earlier

Look for the AMP tag in a start script rather than an end script. The browser should load less content and make fewer requests before redirecting to the AMP version of the content.