Insert C# interface between managed C++ and current C#

This task inserts a layer between current manged C++ interface and C# code and will reside in a new project named Org.Apache.REEF.Bridge.clr. The classes will have the same namespaces and names as the current managed C++ classes. The current managed C++ classes will be pushed down in the namespace hierarchy with an Impl namespace added to the innermost namespace. The new C# classes in Org.Apache.REEF.Bridge.clr will then become the managed interface to the bridge and will call the Impl classes in the managed C++ layer.