Statistics

Initial pull request for statistics feature. 

This includes the ability to generate and store statistics (currently in RavenDB) for command results. 

Statistics get generated and persist like:

	{
	  "Categories": {
	    "CommandStatistics": [
	      "WasHandled"
	    ],
	    "EmployeeStatistics": [
	      "RegisteredEmployee"
	    ]
	  }
	}

The statistics pipeline is visitable by creating a statistics plugin that implements ICanGenerateStatisticsForCommand. 

The resolving of these plugins happens in the CommandStatistics object and is initiated from the CommandCoordinator.