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.

examples/sshd.py

@@ -7,6 +7,7 @@ While something like rshd(8) would be lighter and faster,
 (and perfectly adequate on an in-machine network)
 the advantage of running sshd is that scripts can work
 unchanged on mininet and hardware.
+
 """
 
 import sys ; readline = sys.stdin.readline