Allow chrome storage api

When creating a Chrome app, obviously one should use `chrome.storage.local` or `chrome.storage.sync`. To enable one to use `core-localstorage` on a Chrome app, there should be an option allowing the user to choose which api to use.

This PR includes an attribute `useChrome`, which by default is left as null - allowing normal usage of localStorage.  If `useChrome` is `local`, it will use `chrome.storage.local`.  If `sync`, it will use `chrome.storage.sync`.