Always work with pointers to filters and subscriptions, drop the references for API consistency

include/AdblockPlus/FilterEngine.h

 #ifndef ADBLOCKPLUS_FILTER_ENGINE_H
 #define ADBLOCKPLUS_FILTER_ENGINE_H
 
 #include <vector>
 #include <map>
 #include <string>
 #ifdef _MSC_VER
 #include <memory>
 #else
 #include <tr1/memory>
@@ skipping 79 matching lines @@
   typedef std::tr1::shared_ptr<Filter> FilterPtr;
   typedef std::tr1::shared_ptr<Subscription> SubscriptionPtr;
   typedef void (*SubscriptionsCallback)(const std::vector<SubscriptionPtr>&);
 
   class FilterEngine
   {
     friend class Filter;
     friend class Subscription;
   public:
     explicit FilterEngine(JsEngine& jsEngine);
-    Filter& GetFilter(const std::string& text);
-    Subscription& GetSubscription(const std::string& url);
+    FilterPtr GetFilter(const std::string& text);
+    SubscriptionPtr GetSubscription(const std::string& url);
     const std::vector<FilterPtr>& GetListedFilters() const;
     const std::vector<SubscriptionPtr>& GetListedSubscriptions() const;
     void FetchAvailableSubscriptions(SubscriptionsCallback callback);
     FilterPtr Matches(const std::string& url,
         const std::string& contentType,
         const std::string& documentUrl);
     std::vector<std::string> GetElementHidingSelectors(const std::string& domain) const;
 
   private:
     JsEngine& jsEngine;
 #if FILTER_ENGINE_STUBS
     std::map<std::string, FilterPtr> knownFilters;
     std::vector<FilterPtr> listedFilters;
     std::map<std::string, SubscriptionPtr> knownSubscriptions;
     std::vector<SubscriptionPtr> listedSubscriptions;
 #endif
   };
 }
 
 #endif