Initialize driver only after first API call

As suggested in #114 discussion, _initStorage is called only after the first call to the API.

This way:
1. Config can be set after script load and without using window.localForageConfig global variable (use localforage.config);
2. if setDriver is called to override the default driver selected, anything hasn't been done yet;
3. Load time should be improved if no calls are made to the API.