Added ISignals (Public Interface Signal Wrapper).

This wrapper is used to expose only methods necessary for subscribers.

Example:

   class MyClass
   {
   public:
      Torque::ISignal<void()> getMySignalInterface() { return mProtectedSignal; }
   protected:
      Signal<void()> mProtectedSignal;
   };
Merge pull request #27 from LuisAntonRebollo/Signal_public_interface

Added ISignals (Public Interface Signal Wrapper).