add select_related and partial prefetch_related support

implement support for a single query for select related base fetches across polymorphic models.

adds a polymorphic QuerySet Mixin to enable non polymorphic models to fetch related models.

fixes: django-commons#198 django-commons#436 django-commons#359 django-commons#244
possible fixes:
    django-commons#498: support for prefetch_related cannot fetch attributes not on all child models or via class names
related: django-commons#531

Author: pgammans