python: add example cat.py for bitmap text handling

python/cat.py

+#!/usr/bin/env python
+
+import sys
+import client
+
+# read lines from stdin and write them to the display.
+# includes scrolling.
+# FIXME: scrolling is slow. use a pixel buffer and blitting.
+if __name__ == "__main__":
+    y = 0
+    lines = []
+    line = sys.stdin.readline()
+    while(line != ""):
+        client.write(0, y, line)
+        lines.append(line)
+        if(y == client.NUM_SEG_Y):
+            lines.reverse()
+            lines.pop()
+            lines.reverse()
+            y = 0
+            for l in lines:
+                client.write(0, y, l)
+                y += 1
+        else:
+            y += 1
+        line = sys.stdin.readline()
+