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 list of instances on front page.
Add mixins to store a request's instance on a form save, and to restrict
displaying of items.
Add helper classes that other apps' tests can use.
Add instance title and description.
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.
Add field for creator of an instance.
Allow project to override instance app urlconf, and add docs.
Set cookie domain so you're logged in to all your instances, and move admin to host domain only.
Add middleware variable for if user has access to edit instance.
Override password hash to speed up tests.
Add form for editing title/description of an instance.
Initial packaging stuff.