Find the unknown host into network.

host discovery module: ryu/topology/hosts_discovery.py
host event module: ryu/topology/event_hosts.py
host REST API: ryu/app/rest_hosts_discovery.py

eg:
$curl http://localhost:8080/hosts
[{"peer_mac": "00:00:00:00:00:04", "port_no": "2", "dpid": "4"}, {"peer_mac": "00:00:00:00:00:02", "port_no": "2", "dpid": "3"}, {"peer_mac": "00:00:00:00:00:01", "port_no": "1", "dpid": "3"}]


Signed-off-by: Samuel Deng <mr.dengshuoling@gmail.com>
delete some debug info in rest_topo.py
add .gitignore to ignore .egg
add topo_test.sh to start test script.

Signed-off-by: Samuel Deng <mr.dengshuoling@gmail.com>
Merge branch 'master' of https://github.com/samueldeng/ryu

Conflicts:
	ryu/app/rest_hosts_discovery.py