Case sharing assertions

http://manage.dimagi.com/default.asp?72641#463147

Depends on https://github.com/dimagi/commcare-translations/pull/6 for proper UI display.

Includes functionality and unit tests.

Passed QA: https://docs.google.com/a/dimagi.com/spreadsheet/ccc?key=0An6SJU-bhyxudGtrdDdUV2FhN0lVUXp2OXVvbGlQb1E&usp=drive_web#gid=3