avoid unnecessary gen_freq_bits override

grayscale.py

@@ -6,9 +6,7 @@ from sstv import SSTV, byte_to_freq
 
 class GrayscaleSSTV(SSTV):
 
-    def gen_freq_bits(self):
-        for item in SSTV.gen_freq_bits(self):
-            yield item
+    def gen_image_tuples(self):
         for line in xrange(self.HEIGHT):
             for item in self.horizontal_sync():
                 yield item

sstv.py

@@ -97,6 +97,11 @@ class SSTV(object):
         parity_freq = FREQ_VIS_BIT1 if num_ones % 2 == 1 else FREQ_VIS_BIT0
         yield parity_freq, MSEC_VIS_BIT
         yield FREQ_SYNC, MSEC_VIS_BIT  # stop bit
+        for freq_tuple in self.gen_image_tuples():
+            yield freq_tuple
+
+    def gen_image_tuples(self):
+        return []
 
     def horizontal_sync(self):
         yield FREQ_SYNC, self.SYNC