option: fix bogus RST role
doc: add more references
httputil: add module

This is for experimentation and comparison between Python HTTP server
interfaces and CoAP interfaces.
http_server: add blackboard resource
http_server: add block resource
option: add content format constants and media type map
endpoint: refactor to separate communications support

Introduce LocalEndpoint which defines the interface for message-layer
communications, and SocketEndpoint which extends it to hold a locally
bound socket.
endpoint: rearrange to expose upcoming content change
endpoint: refactor cache infrastructure

The interface we use now de-emphasizes direct access to the queue in
favor of a process_timeout() operation that manipulates the queue
implicitly.
endpoint: document cache entry process_timeout, refine creation interface
endpoint: add statistics

To verify we are meeting congestion requirements we need to know when we
last heard from an endpoint, and how much we've sent it since we last
heard from it.  For analysis it's also good to know overall how much has
been sent to and received from the endpoint.  Support all this at the
level of raw send and receive.
endpoint: refactor statistics and remote state

Move message ID management down to LocalEndpoint, and support a normal
practice where the message ID is assigned when the message is
transmitted.  Also collect all statistics and state that are relevant to
a remote endpoint in an association class that links the source endpoint
to that remote endpoint, since the source endpoint is also relevant to
the state.
endpoint: use more evocative parameter name
endpoint: rework cache to support pending entries

Congestion management may require that we hold a transmission until
there are fewer than NSTART outstanding interactions.  These entries
still need to be in the cache since their MessageID is reserved, but
they should not start their lifecycle until they have been activated by
assigning them a time-due.  While inactive they remain in the cache
pending list instead of the priority queue.
resource: add module
message: raise exception on unrecognized critical option