Adding a field to 'event' to record which organization organized a workshop.

We need to track which organization did the admin for a workshop (e.g., SSI organizing in the UK, MSL organizing in North America) so that fees can be divided up.  We do this by having another foreign key from the `event` table back to the `site` table, and adding the partner organizations as sites.
Adding field to record admin fee for workshop.

We now collect an administration fee for most workshops, but the
amount may vary, and we may waive it entirely.  This change adds
a field to record that amount.
Merge pull request #43 from gvwilson/adding-organizer-field-to-event

Adding a field to 'event' to record which organization organized a workshop.
Updating description
One logo per page is enough
First thing's first, start with some failing unit tests.
Added query methods for past, ongoing and upcoming events.

The Event object now has three additional methods:

Event.objects.past_events() is used to access past events.
Event.objects.ongoing_events() is used to access ongoing events.
Event.objects.upcoming events() is used to access upcoming events.

All of these return querysets, which can be filtered further if
necessary.
Make test events into a fixture and mock datetime.date.today so
that future events are always in the future, past in the past
and ongoing ongoing.
Add a failing test to the landing page to check for upcoming events.
List upcoming events on the index page.

I also wrote the get_absolute_url method for Event objects
so that we can easily fetch their url.
Added the sites model to the django admin, and link to
the django admin page from the all_sites view if the user
is logged in and has the correct permissions.
Define tests programatically, and make custom calls chainable.
Merge pull request #45 from rbeagrie/master

List events on landing page (fix for issue #1)
Adding command to Makefile to re-run tests
Merge pull request #47 from gvwilson/adding-target-to-run-tests

Adding command to Makefile to re-run tests
Create and edit sites using django generic views.
Merge pull request #46 from rbeagrie/add_site_link

Added the sites model to the django admin
Merging
Adding editabilit to training cohorts
Adding Bootstrap styling.

1.  Generic page layout now references Bootstrap files delivered via CDN.
2.  Page layouts now use Bootstrap CSS classes.