Add Section#start_date

This is a virtual attribute that resolves to the :starts_at attribute of
the first section part, when ordered by :starts_at.

TODO:  This still needs to be modified to filter out parts in the past.
Merge branch 'master' into order-section-list-by-date
Display available sections chronologically

Added a :starts_at virtual attribute to the Section model which pulls
the first :stars_at date from its parts.  Then we order the
@available_sections for display on the course#show page using this
virtual attribute.
Merge branch 'master' into order-section-list-by-date
Fix sections controller

The sections#index action uses the same _available_sections partial,
which was modified to use an @available_sections instance var.  Need to
set this in the controller action method.
Merge pull request #7 from cit-jmu/order-section-list-by-date

Order the course section list by date