don't choke on rebuilding cases that have been rebuilt before
Merge pull request #220 from dimagi/rebuild-choke

don't choke on rebuilding cases that have been rebuilt before
fix 500 on cases with children that have no opened_on
Merge pull request #221 from dimagi/fix-child-no-opened_on

fix 500 on cases with children that have no opened_on
Merge branch 'master' into pretty-user-display