Initial creation of Instances model and handling.

This stores a DNS label field, and adds some middleware to detect if
you're on a subdomain or not and direct to different urlconfs. It sets
request.instance to be used by the views and templates. If given a
non-existant subdomain, it redirects to the main URL.
Add instance fields to all speeches models.
Specify popit arguments on command line, rather than in settings.
Rename BASE_URL to HOST, and add PORT.
Add list of instances on front page.
Add a form mixin to store the request's instance on all the form saves.
Some more places instance needs manually setting.
Update tests with knowledge of instances.
Add instance title and description.
Add InstanceManager with for_instance method.
Restrict displaying to current instance with a mixin, and subsequent related changes.
Be sure to restrict InstanceFormMixin to the right objects too.
Specify migration must take place before title is added to instances.
popit_url only unique per instance.
Add user account permissions for instances.

This adds a many-to-many table between instances and users, so you
can assign which users can edit which instances.