openflow: Improve docstrings
openflow: Fix redundant event spec

FlowRemoved was listed twice in the list of events exported by Connection
and OpenFlowNexus.  Fixed.
of_01: Fix wrong name of error handler

This addresses a regression stemming from b25ad0047 / ec2053a19 and is
based on a bug report and patch from Peter Peresini on pox-dev.  (The
original patch missed that the bug appeared in two places.)
of_01: More robust handler-setting

While the previous commit fixes a specific bug of a mismatched handler
name, this commit attempts to make the matching of handler to OpenFlow
message more robust to such errors in general.  It uses the handler name
to find the OpenFlow message type rather than the reverse.  (This is the
technique I proposed on pox-dev in response to the initial bug report.)

Thanks again to Peter Peresini for reporting the initial bug.
openflow: Support OFPT_GET_CONFIG_REPLY

This adds a new event for replies to GET_CONFIG requests.  It also stubs
out (but doesn't implement) the handler for the sole remaining OF 1.0
message we don't do anything with.