packet lib: add LLDP support.

See "ryu/tests/unit/packet/test_lldp.py" to use this library.

This patch is based on Yamahata's topology discovery patch series.
http://thread.gmane.org/gmane.network.ryu.devel/467

Signed-off-by: YAMADA Hideki <yamada.hideki@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>