make easier to create benchmarks with default regen frequency

Hey Alex,

I was trying to configure regeneration of the data for my sorts benchmarks, and I've found it would be easier if I override default frequency for PeriodicReinstantiation trait

so my code looks like this

    object SortBenchmark extends PerformanceTest.Quickbenchmark {
      override def measurer = new Measurer.Default with Measurer.PeriodicReinstantiation {
        override val defaultFrequency = 1
      }
      // bunch of performance tests
    }

I would be glad to hear your feedback.

Thanks,
Eugene