core: Allow no callback for call_when_ready()

This lets you register dependencies without actually having a
callback.
l2_nx: Fix up initialization

The launch() function before was just a quick hack; this fixes it up a
bit to actually handle dependencies.
openflow.nicira: Improve initialization

This makes the order of initialization sane.
nx: Minor cleanup / improvement
nx: Add OFPR_INVALID_TTL

Technically, this is part of OpenFlow 1.1, but it's useful with OVS, and
since we don't have OpenFlow 1.1 support yet, it's in here at least for
the time being.
nx: Attemping to fix a couple unpack functions
nx: Untested attempt to add role selection stuff

This adds controller role selection stuff to the nx component, but is
currently untested.  Once it's debugged and the datapath is rewritten to
use it, the older nicira_ext.py should be removed.
of_01: Add eth_addr property to Connection object

This uses the DPID to figure an Ethernet address, as suggested by the
OpenFlow spec.  An alternate approach might be to use the local port's
address, but I don't think we're guaranteed to have one.

There are multiple places where we could refactor code to use this
instead of a helper function.

This also fixes a latent bug since EthAddr wasn't imported.
arp_helper: Include in_port in event
lib.addresses: Support for multicast IP addresses
tests: Improve lib.addresses tests
nx: Add set_tunnel/set_tunnel64 actions

Untested.
nx: Add new flow-based tunnel match entries

See OVS commit 0ad90c845b
nx: NXT_SET_ASYNC_CONFIG support
of_tutorial: Credit Brandon

I think Brandon did the original NOX tutorial.  The POX one is
based on this (and sort of on l2_learning), so he now gets
proper credit.
l2_nx: Improve module docstring

This clarifies that you must have packet-in conversion enabled.
of_json: Improve match->dict conversion

The nw src/dst fields were getting put in the dictionary as
"get_nw_src" and "get_nw_dst".  These are the getter names --
the actual fields shouldn't have the "get_" prefix.  Now
they don't.  (Hopefully.)
of_json: Filter wildcarded IPs in dict conversion

Entirely wildcarded IPs were showing up in the dict conversion.
Now they don't, which is consistent with the rest of the fields.
ip_loadbalancer: Improve comments
ip_loadbalancer: Remove dpid_to_mac() function

This functionality is now available as the eth_addr attribute
of connection objects, so we now use that.