Adding support for getting SharedPreferences from PreferenceScreen

It's now possible to register as a ```OnSharedPreferenceChangeListener``` in a ```PreferenceActivity```, like this:

```
getPreferenceScreen().getSharedPreferences().registerOnSharedPreferenceChangeListener(this);
```

The shared preferences are stored for later use, like the Android SDK. Making it easier to test adding and removing listeners in the activity life cycle.

Added support for loading title and summary from resources in ```Preference``` as a consequence.