#264 - using Apache ObjectSerializer.

This is for #264 - support for Set<String> objects in SharedPreferences.

Some questions:
- This implementation is not very efficient for ApiLevel > 11; mainly because I'm not sure how AA could support this? `androidannotations-api` links against 1.6.
  - Access to `SdkVersionHelper.getSdkInt()` should be ok I guess
- It's using the ObjectSerializer class from the apache pig project as discussed on SO: http://stackoverflow.com/questions/7057845/save-arraylist-to-sharedpreferences
  - A alternative implementation could be to put each entry of the Set into it's own property, like "$IDX_$NAME" along with a "MAX_$NAME" property. This would require a little more work in the AbstractPrefEditorField functions