Merge pull request #17 from changbl/working

Fix bug: OFP_DEFAULT_MISS_SEND_LEN=128 is too small
Finish DHCP

The port on which virtual switch connects to DHCP server is named as "eth_dhcp"
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