Extension for port statistics

We need to create a resource extension to support retrieving statistics for a given port. The new resource will be named "stats" and callable via:

GET /v1.0/extensions/ovs/tenants/<tid>/networks/<nid>/ports/<pid>/stats

Any plugin supporting this extension should list "ovs_portstats" in their supported_extensions and should implement the a get_port_stats() method which returns the following:

{
  "rx_packets": 0,
  "rx_bytes": 0,
  "tx_errors": 0,
  "rx_errors": 0,
  "tx_bytes": 0,
  "tx_packets": 0
}