We specifically use relative package references where possible so this demo
works even if the user names their package (in the '$VENV/bin/pcreate -s
zodb ...' step) something other than 'tutorial'.

Specifically:

- use relative imports
- use plain relative URLs for resources (like stylesheets and images) in
  page templates.

Direct uses of the package name, like in __init__.py 'config.scan()'
statements, are already adjusted by the paster/pcreate, so we don't have to
worry about them.