Implement group table

* Still need to add actions to send to group table for L2 broadcast/multicast
* Still need unit test for group table

Change-Id: I062436fa22ebd5703424f96b22098c5801c0ae4b
Signed-off-by: Rob Adams <readams@readams.net>
Most basic features now working, including:
    * Bridging and routing within and across endpoint groups
    * Respond to ARP sent to the virtual router IP
    * Add nicira extension support where possible/required.
      * Sets up Nicra group actions in the group table to set tunnel
        destination and tunnel ID
      * Sets sepg, scg, depg, dcg, fd, bd, and vrf into registers
    * Enforces policy configured by contract
    * Adds a simple test script to test OF overlay with mininet

Change-Id: Ieb4633a529d529c4b3586bc8cd6d046bcad84725
Signed-off-by: Rob Adams <readams@readams.net>