Actually fixes `QUOTA_EXCEEDED_ERR` error in private Safari window.

@alexgorbatchev's fix in #24 is almost, but not quite, complete. Safari will still throw an exception:

```error
TypeError: 'undefined' is not a function (evaluating 'webStorage.setItem('ngStorage-' + k, angular.toJson(v))')
```

...because we explicitly set `webStorage` to an empty object `{}` earlier.

See #24 for more information about this bug, which can be reproduced only in OSX Safari or iOS Safari running in "private browsing" mode.