handle window.localStorage being unavailable

Some security settings (Firefox 'always ask' cookie setting) may result in accessing window.localStorage raising SecurityError.

Setting this as the default, even if unused, makes it impossible to instantiate a Tour in such a context.

This is related to #236, where the proposed solution doesn't work because

```javascript
tour = Tour({storage : false});
```

will still raise a SecurityError when the unused default is populated.

This changes the default to `false` if accessing localStorage fails.