Add DjangoPJAXMiddleware and tests.

It has two jobs. On the request phase, it strips the _pjax GET parameter from both request.META['QUERY_STRING'] and from request.GET. On the response phase, it adds the X-PJAX-URL header so that jquery-pjax correctly handles redirects.
Remove unnecessary try-catch.

If "pjax_captured_blocks" is not in context.render_context, that's a bug and we shouldn't suppress it.