Expose instantanize function publically.

Motivation: 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.