Now, to reference mininet files, use 'import mininet.mininet'.

PYTHONPATH mods are no longer required for installation.

Added commentary on control network to mininet.py.
Hopefully fixed linearbandwidth.py for real.

Not sure if I want nox_core -v in nox.py - it's useful for debugging.
ripcordtest.py (and grid.py) still depend on routing, which I haven't been
able to test yet.

instantiate a mininet Network based on its topology.

Also minor cleanup of nox.py and GridNet class in mininet.py.

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.

low-level primitives.

The "nox.py" example attempts to use nox as a controller rather than
controller(8).

More edits for INSTALL and README, and refinements for sshd and xterm
examples.