Fix bugs and redesign data structures

-- Fix bugs in _handle_arp_request:
Wrong ports for broadcast

-- Fix bugs in _handle_arp_reply:
Wrong datapath to forward the reply

-- dpid_ip_to_port redesign:
Rename to dpid_to_conns
New data structrue: {dpid -> {ip address -> port}}
Merge pull request #4 from powerist/working

Fix bugs and redesign data structures