Vapour: changed window._timer to use vapour.doc as cache

1. This moves the element search context one level up so that plugin
   elements in the `<head>` can also be found (e.g. mobile favicon).
2. Saves the creation of another `$( document )` jQuery object.
3. Saves the logic required to check if the window._timer _cache has been
   initialized.
Favicon: initial commit of the favicon plugin

1. Uses `<link rel="shortcut icon">` as its plugin element.
2. Default behaviour is to add a mobile favicon to the page if a
   shortcut icon is defined.
3. Also provides events to update the mobile and shortcut favicons.
Merge pull request #3137 from patheard/mobile-favicon

Mobile favicon plugin