Initial support for automatic update

Still to do:

- The `Update Manifest URL` needs to be set
- The `Developer Identifier` needs to be set (see [here](http://developer.apple.com/library/safari/#documentation/Tools/Conceptual/SafariExtensionGuide/UpdatingExtensions/UpdatingExtensions.html#//apple_ref/doc/uid/TP40009977-CH12-SW1)).
- Set the download url of the extension (no zipped) with an appropriate mime:
    - There was a web service which allowed to set the MIME for the raw GtiHub and for the moment we could use it but, I don't remember the name :-(.

> Your-.safariextz-URL must be a valid URL to download the current version of your extension.
Be sure your web server has the .safariextz file extension associated with the MIME type 
application/octet-stream. For more information, see “Putting Your Extension on a Web Server.”