Add fromSharedPreferencesChanges Observable

This is wrapper around SharedPreferences.OnSharedPreferenceChangeListener 
It's pretty simple and straightforward, but is useful to observe SharedPreferences changes.

About `subscriber.isUnsubscribed()` check:
I was just following instructions from https://github.com/ReactiveX/RxJava/wiki/Implementing-Your-Own-Operators#other-considerations.

Sometimes I'm not sure should I check for it or not.