Adding a isEmpty to the Store interface

For the iOS library, we got feedback that a "isEmpty" method would be handy on the AGStore protocol (see AGIOS-32 for more details).

The Java interface does also not have a method like that:
https://github.com/aerogear/aerogear-android/blob/master/src/org/jboss/aerogear/android/datamanager/Store.java

It would be an easy fix to add this method