Add support for ARP-based host discovery

This pull request adds a new stage to the OpenFlow 1.0-based controllers that will inspect ARP packets sent to the controller and record the switch and port id where that packet was first seen. This code assumes that the entire network is managed by the controller, and that no MAC/IP address will appear at multiple ports, and is documented as such.

This is an optional stage that is not installed by default.

Currently, the information stored in the "NIB" cannot be queried effectively. That is a task left for after #91 is resolved.

This pull request depends on #92.

Closes #89.