Pass active backend to index queryset calls (closes #534)

Now the Index index_queryset() and read_queryset() methods will be called with
the active backend name so they can optionally perform backend-specific
filtering.

This is extremely useful when using something like Solr cores to maintain
language specific backends, allowing an Index to select the appropriate
documents for each language::

    def index_queryset(self, using=None):
        return Post.objects.filter(language=using)

Changes:
    * clear_index, update_index and rebuild_index all default to processing
      *every* backend. ``--using`` may now be provided multiple times to select
      a subset of the configured backends.
    * Added examples to the Multiple Index documentation page