Added 'redcarpet' gem (Markdown support) dependency.
Nested assignments under courses, added url_part and description fields to assignment.
Added event_channel method to all ActiveRecord objects, which can be used to get Juggernaut channel names.
Added bootstrap-datetimepicker.
Added markdown mode for Codemirror and some additional Codemirror styling for forms.
Added first cut of assignment management features.