0.4.0

• Major BC break: A provider is no longer passed a database session, but a
  database session maker. This change was needed to get the provider to function
  properly in threaded web applications. This will mean changing the
  code where you're creating your provider. In the past, you probably called
  a session maker first and then passed the result of this call to the provider.
  Now you should just pass the session maker itself and let the provider create
  the sessions for you.
• Different way of fetching the skosprovider.skos.ConceptScheme
  for a provider. No longer fetches a conceptscheme at provider instantiation,
  but when needed. Otherwise we end up with a possibly very long cached version
  of a conceptscheme.