Robot 36 color correction (#12)

The color sequence for Robot 36 is listed as YCrCb in the SSTV Handbook: http://www.sstv-handbook.com/download/sstv_04.pdf (Table 4.3) and the Dayton Paper: https://web.archive.org/web/20120104184535/http://www.barberdsp.com/files/Dayton%20Paper.pdf

* Corrected inter-channel frequency order for Robot 36 mode.
* Switched Robot 36 channel order to CrCb.

pysstv/color.py

@@ -87,14 +87,14 @@ class Robot36(ColorSSTV):
     C_SCAN = 44
     PORCH = 1.5
     SYNC_PORCH = 3
-    INTER_CH_FREQS = [None, FREQ_BLACK, FREQ_WHITE]
+    INTER_CH_FREQS = [None, FREQ_WHITE, FREQ_BLACK]
 
     def on_init(self):
         self.yuv = self.image.convert('YCbCr').load()
 
     def encode_line(self, line):
         pixels = [self.yuv[col, line] for col in range(self.WIDTH)]
-        channel = (line % 2) + 1
+        channel = 2 - (line % 2)
         y_pixel_time = self.Y_SCAN / self.WIDTH
         uv_pixel_time = self.C_SCAN / self.WIDTH
         return chain(