openflow: Better support for vendor extensions

This is definitely not ideal yet, but there are some sticky
design considerations remaining to consider.
libopenflow: Add condition for flow mod equality

The flow_mod equality check was missing a condition (on the
new data attribute).
openflow: Add nicira component for OVS support

This component is a first stab at supporting some of the
Nicira extensions to OpenFlow as implemented by Open vSwitch.

It's subject to change.  And bugs.
forwarding: Add Nicira-extension learning switch

This is an L2 learning switch that uses Nicira extensions provided
by Open vSwitch.  Mainly, it utilizes multiple tables.