Provide a data store interface to enable customizable persistent storage

[mnelson]

Browser incompatibility is causing data loss in localStorage. We're now using sessionStorage as the primary store. If a failure occurs, we fallback to other stores.

• sessionStorage
• localStorage
• cookie
• backup (a model which implementing apps can observe).

The backup case would look like this:

app.dataStore.backup.observe(function(changeSet) {
 var change = changeSet[0];
 console.log("Key: " + change.name + " Value: " + change.value);
});

[smmullen90]

+1