Mobile favicon plugin

Includes the following 2 commits for #3087:

**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 mobile favicon plugin**

Uses `<link rel="shortcut icon">` as its plugin element.  If this exists on the page, a mobile favicon is added to the `<head>` using the same path as the existing favicon.