Make JsEngine::Evaluate() return a wrapper for v8::Value to accessdifferent variable types easily

include/AdblockPlus/JsEngine.h

 #ifndef ADBLOCKPLUS_JS_ENGINE_H
 #define ADBLOCKPLUS_JS_ENGINE_H
 
 #include <stdexcept>
 #include <string>
 #include <v8.h>
+#include <AdblockPlus/JsValue.h>
 
 namespace AdblockPlus
 {
   class ErrorCallback;
   class FileReader;
   class WebRequest;
 
   class JsError : public std::runtime_error
   {
   public:
     explicit JsError(const v8::Handle<v8::Value> exception,
         const v8::Handle<v8::Message> message);
   };
 
   class JsEngine
   {
   public:
     JsEngine(const FileReader* const fileReader,
              WebRequest* const webRequest,
              ErrorCallback* const errorCallback);
-    std::string Evaluate(const std::string& source,
+    JsValuePtr Evaluate(const std::string& source,
         const std::string& filename = "");
     void Load(const std::string& scriptPath);
     void Gc();
 
   private:
     const FileReader* const fileReader;
     v8::Persistent<v8::Context> context;
   };
 }
 
 #endif