Expose instantanize function publically

InstantClick currently doesn't support any kind of 'live' binding of elements, e.g. if you add <a> elements to the DOM after InstantClick has been initialised, those elements will not be correctly set-up as InstantClick-enabled.

Exposing instantanize() means one can enable InstantClick for elements added after InstantClick.init() - by calling InstantClick.instantanize() after elements have been added to the DOM.