Address properties

[gdamore]

Implement property support to get / set addresses (remote and local).

[gdamore]

See also #40 -- please keep this in mind -- we think these address properties should be via nng_sockaddr. Note that this means that the compatibility layer will need to convert these to and from string representation.

[gdamore]

So we have a few things.

a) URLs are strings. We have to support these for configuration, and we also support obtaining the URL from a pipe as a string. The URL does not give details about the peer, but merely the service address used.

b) Pipes have a readonly nng_sockaddr pair -- one local and one remote, that can be obtained as options. This provides details about the actual TCP, inproc, IPC, or ZT connection. This may differ from the URL -- for example locally selected ephemeral ports are reported, etc.

Right now only TCP support for this is missing, but that's about to be rectified.