add a benchmark "runner" to ignore setUp and tearDown time

Hi,

I did a small development to exclude the setUp and tearDown time of the benchmark time.
Roughly the idea is to override the default runner by a customized one (by adding the annotation @RunWith(BenchmarkRunner.class)). The test clock is started and stopped by a call back at the test level.
I added a new unit test : TestSetUpTearDown.java

Is it possible to integrate this development in the 0.8 version ?

Thanks
Antoine