Add prefix configuration via ngStorageProvider

Now one can set his own prefix for storage keys via ngStorageProvider

closes https://github.com/gsklee/ngStorage/issues/26 and https://github.com/gsklee/ngStorage/issues/17
This readme is actually incorrect because it needs to be $localStorageProvider and $sessionStorageProvider. Not sure if it was intended for copy/paste, but I tried it.

Other than that, this code works amazingly.