Pickerbug demo rails app

To investigate a bug.

See activeadmin-plugins/active_admin_datetimepicker#20

Ruby 2.1.0, Rails 4.0.13

See .ruby-verison and Gemfile

Uses sqlite for now, because I see no reason not to for this little demo

I installed active-admin with --skip-users, since this isn't a real production-intended app.