Add CompetitionBase: now competitions can be created in form of unit test.

As discussed in AndreyAkinshin/BenchmarkDotNet#2 I've created a integration between BenchmarkDotNet and NUnit.

Now new competition can be created as a class inherited from `CompetitionBase`, each task should be represented by one method marked with `BenchmarkMethod` and returning `Action` delegate.

Classes inherited from `CompetitionBase` are recognized as tests (with single test method) by ReSharper and NUnit and can be executed either inside VisualStudio or via external test runner.