event.py: add link events.
switches.py: add link discovery (only of1.0).
dumper.py: add handler for link events.
TODO: support other OpenFlow version.
Signed-off-by: YAMADA Hideki <yamada.hideki@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>