Clean up.

Made the demo (yoursite) directory a basis for creating new sites
by using an INSTANCE_NAME variable in settings.py and wsgi.py to
replace the hardcoded 'yoursite'. This is done so that when creating
new sites, we can just copy s13sitedemo to another directory and
simply generating a new SECRET_KEY for the new settings.py file.

In line with this, the templates and static files have been moved
from s13sitecore to s13sitedemo so that it is easier to find and
to edit those files (per website) when necessary.