A simple mobility API and example

Tricky but not impossible: create a `moveHost()` API for simply moving a host across switches.

Make sure that you:

* Rename the interface with its appropriate canonical name
* Preserve the correct OpenFlow port mapping

I have been thinking about moveIntf() for awhile, and I'm still not sure if I like it. We should ship this interface as part of the example, and then maybe revisit it later.