- Changing the scenario to include superstepNo and changing vertexValue to vertexValueBefore and	vertexValueAfter.
- Fixing TestCaseScnearioWriter	   and GiraphScenarioWrapper according to the .proto change.
- Adding initial AbstractInterceptingComputation class and DebugConfig and an example SimpleShortestPathsDebugComputation. (currently does not save anything as a .proto though some code is initiated to do so.)