support displaying the actual queries that failed the limit

[zsoldosp]

current workaround to achieve this is

            from django_performance_testing.signals import results_collected
            def dump_queries(sender, results, context, **kwargs):
                try:
                    write_result, = (r for r in results if r.name =='write')
                    if write_result.queries:
                        print '\n'.join(q['sql'] for q in  write_result.queries)
                except ValueError:
                    pass
            results_collected.connect(dump_queries)
            with QueryBatchLimit(write=0):
                ....