Add created_at to json responses for tasks, users, photos, field_reports

We said it was only necessary for photos and field_reports, but I think tasks and users probably want to know too. For users, we can say something like: "Watersheding since Nov 2014" or something more professional like "Member since Nov 2014"

For tasks, a created at date is good for knowing how old it is.

Fix #325 
Fix #326