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

Author: pgammans