of1.3: Add special group identifier values defined on OpenFlow 1.3.1

This patch add missing group special values of OpenFlow 1.3, added to
the 1.3.1 version.

It is useful to create a flow_mod message with an out_group with no >
group restrictions and for flow_stats messages.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
doc: fix the mailing list address

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
add a threading hub module

this provides gevent-like api using eventlet.
sweep the tree to change from gevent to ryu.lib.hub

mostly mechanical changes.
also, change the requirement from gevent to eventlet.
ovs.poller: workaround a bug in eventlet
controller: fix send queue draining again

and this time add comments to explain the intention.
limit the size of ryu app's event queue

otherwise a queue grows too long and consumes much memory on load.
the size used in this commit (128) is arbitrary.
vsctl test code from yamahata
ovs_vsctl.py: rebase and tweaks
ovs_vsctl.py: print timing info
add a ryuapp to provide python interactive console for debugging