Merge pull request #18 from powerist/working

Finish DHCP
Fix an ARP bug

In setup_fwd_flows(), when switch_id and peer_switch_id are the same,
need to setup only one flow instead of two.

Also the interface of setup_fwd_flows() is simplified
Merge pull request #19 from changbl/working

Fix an ARP bug
Style polishing in DHCP