Index index_queryset method can see the target backend

This allows backend-specific logic - for example, with language-specific
backends an index can do something like

    def index_queryset(self, using):
        return qs.filter(language=using)
Make using optional for both index_queryset and read_queryset
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
Tests: use a true tempdir for multipleindex Whoosh storage

This ensures that there's no way for results to persist across runs
and lets the OS clean up the mess if we fail catastrophically
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
Fix typo in multiple_index docs
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