Add django-adminplus dependency [requires virtualenv update]

With the help of django-adminplus one can add custom views to the
Django admin interface that are not tied to modules. This is a good
way to bring new tools to the admin interface (e.g a data importer).
Add simple urljoin method to combine two URL parts
Importer: add basic admin importer view
Importer: make filter work to selectd only known projects
Importer: sort project listing
Importer: make project selection list look nicer
Importer: use proper inital values for project list
Importer: add documentation
Importer: add optional URL, zoom level and file extension in YAML

If a stack in a project file now contains a 'url' field, it is used as
the stack's image base. If done so, the fields 'zoomlevels' and
'fileextension' need to be available as well. If no 'url' field is
present, the importer looks for the 'folder' setting. If there are then
still 'zoomlevels' and 'fileextension' fields around, those will be used
as well. The importer will retrieve this information only if one of them
is missing.
Importer: add support for overlays

In a project file, each stack can now have a set of overlays
attached. In terms of declaring a data source, overlays behave very
similar to stacks, but don't need zoom level information. One can
 define a default opacity if wanted (if not, it defaults to zero).
Also, the confirmation view now displays all the overlays for each
stack.
Merge pull request #460 from tomka/admin-import-view

Importing tool for admin interface