models: Add OnCall.description for position clarification

My conversation with Jordan Schatz from 2013-11-12:

  Jordan:
    Can we add more descriptive text to the page it opens [the list
    page]?

  Trevor:
    Sure thing, just tell me what to drop in the template.

  Jordan:
    ...  A paragraph that helps users distinguish between network
    issues which go to Steven and beehive issues that go to us.

I also switched to <dl> (from <table>) in list template, because the
slug and get_on_call results are likely to be short while the
description may be a good deal longer.  This usage also matches the
dictionary term/description semantics pretty well, although I'm
cramming the get_on_call results into the "term" portion.

I also added "on-call-{{on_call.slug}}" ids to the <dt> tags so you
can link to a specific entry in the list (although in that case you
may just want to link to the entry-specific page).

Note that without South (or some other migration tool), you'll have to
either migrate existing databases by hand or drop the django_on_call
table and recreate it with syncdb.
Bump to v0.5

Add OnCall.description (requires a manual migration).

Changes since v0.4:
* models: Add OnCall.description for position clarification