Add TypeFinder class to find subtypes that implement the contract

[DevD4v3]

This class allows to search for the subtypes that implement the contract from a plug-in.
It contains only one method:

public static class TypeFinder
{
   static IEnumerable<TSupertype> FindSubtypesOf<TSupertype>();
}

This method will search for classes that implement the contract specified by TSupertype (a generic parameter) and returns the instances that implement the contract.