Adds references and branches slides

Mostly adds slides about references and branches. Also tweaked python
start code and re-aranged / tweaked some existing slides

code-examples/python-start.py

@@ -1,6 +1,7 @@
-import sys
-    def main():
-        print("Hello")
+#!/usr/bin/env python
 
-    if __name__ == "__main__":
-        main()
+def main():
+    print("Hello")
+
+if __name__ == "__main__":
+    main()