SD File object implements Stream.

Added peak() and available() using a single byte buffer.
Added flush().

cores/arduino/Stream.h

@@ -28,6 +28,7 @@ class Stream : public Print
   public:
     virtual int available() = 0;
     virtual int read() = 0;
+    virtual int peek() = 0;
     virtual void flush() = 0;
 };