Reintroduce BUrlResult and add BDataRequest

* BUrlResult is back, with ContentType and Length methods.
* BHttpResult subclasses it and use HTTP header fields to implement
those
* Introduce BDataRequest for "data" URIs. These embed the data inside
the URI, either as plaintext or base64 encoded.

headers/os/net/FileRequest.h

@@ -19,10 +19,13 @@ public:
 									BUrlContext* context = NULL);
 	virtual						~BFileRequest();
 
+	const 	BUrlResult&			Result() const;
             void                SetDisableListener(bool disable);
 
 private:
 			status_t			_ProtocolLoop();
+private:
+			BUrlResult			fResult;
 };