Performance improvement by limiting calls to localStorage.getItem().

The `get()` method of the `$translateLocalStorage` factory is called many times, so save the language key in memory and serve it from there.

Here's a demo of how often get() is called even if there's very few strings:
http://plnkr.co/edit/nlytP5RPIeIzfJ6ZhoPU?p=preview
(Look in JavaScript console.)