First crack at allowing Controller to be customized.

Network may now be used with custom controllers.
An example of doing this is in nox.py, which instantiates at TreeNet
using a custom Controller, NoxController, that runs nox_core rather
than the reference controller.

INSTALL

@@ -1,6 +1,8 @@
 Preliminary Mininet Installation/Configuration Notes
 ---
 
+- This is not (yet) a 'release'; things may be broken.
+
 - Mininet is not currently 'installed.' If you want to install it,
   so that you can 'import mininet', place it somewhere in your
   python path.