Added help text for TwitterUser.date_last_checked, and for TwitterUserItem.people, .words, .locations.  Fixes #108.
Added filter_horizontal (nicer multi-selector) for TwitterUserAdmin.sets, made all fields on TwitterUserItemAdmin, -ErrorAdmin, and -TimelineJobAdmin read-only. Fixes #108.
Merge pull request #144 from gwu-libraries/t108a-admin

T108a admin