Added multi-index examples for backend-based content filtering

This demonstrates how to filter index update / read querysets when
necessary and two examples for how to perform this filtering
Index.update: pass using through to index_queryset() call
Basic tests for filtering index_querysets based on using value
PEP-8
update_index: update all backends by default

* Change the default behaviour to update every backend
* Allow --using to be repeated to update multiple named backends
clear_index: adopt new behaviour of update_index

* By default, all backends are cleared
* --using may be specified multiple times
Basic tests for management commands with --using

This ensures that the management commands correctly process --using options and
pass them to the appropriate lower-level code