controller.handler: g/c now unused code

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
controller.handler: use normal classes rather than a namedtuple

and add some comments.  no functional changes.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
controller.handler: simplify code

and use consistent variable names.  no functional changes.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
RyuApp: add a method to unregister a handler

this method undos register_handler.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
RyuApp.get_handlers: restructure code and add a comment

no functional changes.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
RyuApp: allow observing events without having handlers

this will be used by ofctl app which registers handlers dynamically.
when unobserving an event cls dynamically, there might be events of
the cls left in app's event queue already.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
RyuApp: add methods to observe/unobserve specific event dynamically

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ofp_event: add a function to find Event class for given OFP class

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ryu.app.ofctl: implement reception of reply message

an example:
    msg = parser.OFPEchoRequest(datapath=datapath)
    result = ofctl.api.send_msg(self, msg, reply_cls=parser.OFPEchoReply)
    assert isinstance(result, parser.OFPEchoReply)

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ryu.app.ofctl: implement multipart

an example:
    msg = parser.OFPPortDescStatsRequest(datapath=datapath)
    result = ofctl.api.send_msg(self, msg,
                                reply_cls=parser.OFPPortDescStatsReply,
                                reply_multi=True)

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
ryu.app.ofctl: raise exceptions on openflow error messages

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
RyuApp: delete a key without handlers when unregistering a handler

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>