start new development circle, point to web page
enhance error handling with missbehaved sites and resources
hybrid stack support for proxy

this affects the mess of getaddrinfo, but doesn't make it worse
simple proxy demo
introduce network dumping

this allows analyzing network communication more easily than using the
debug output.
forward proxy: fix error message
coap-forward-proxy: generalize proxying

this generalizes the coap-forward-proxy into a generic proxy (which
can't be configured on the command line yet) as a part of experimenting
with proxying in general.
rename coap-forward-proxy to coap-proxy
introduce unresolved_remote in message

this is a way for a message creator to specify the message's destination
in an intermediate way: not as semantically correct as setting the
Uri-Host option (which wouldn't allow implementing proxying), and not as
low-level as setting the `remote` property (which is difficult to get
right anyway).

as a result, the ProxyForwarder could be drastically simplified. note
that its constructor api changed, as host:port splitting is now handled
by the protocol.
use unresolved_remote in coap-proxy
coap-proxy: fix forward mode
coap-proxy: command-line configurability
fix leftovers from 62f22cfe
fix regression w/rt client-side observations

the .observation property came live only until some time into the
request, this just makes things needlessly difficult.
rework of resources

the original txThings resources were modelled after twisted concepts i
am not familiar with and that were not flexible enough in the asyncio
port when it came to where the application can do async stuff.

this unifies the resource and site concepts -- the server context just
gets a resource that will have its .render(request) method called. if
that resource is a 'Site' that dispatches the request to other
resources, that's fine with the context but none of its concern.

as resources are not necessarily persistent any more in this concept,
the way resources interact with persistence was changed here.
implement the new server side observations

this is a result of the interface change in the resources. it does
several things:

* register the observation with the context. this is needed because same
  endpoint/token combos should clear old observations.
* simplify the Responder, as it now does not need to distinguish between
  get_resource and render phase
* concentrate responsibility for registering and unregistering in the
  ServerObservation objects
server.py: adapt to the new way of creating resources
tests: adapt to the new way of creating resources
rename CoAPResource to Resource
resource documentation