Rewrite Ripcord test and most of mininet

Make much of the Mininet code object-oriented around the new Mininet
object, and create a generic way to build a Mininet.

Previously, each network topology was created by Mininet API calls,
which seems much less approachable for new users than passing in a
topology object along with controller, switch, and host classes.

The modified code is all in ripcordtest.py, but will need to replace the
main mininet.py code eventually.

Kernel-space support only, for now.