network: Respond to messages before closing connections

This makes a connection tell the actor to stop the connection, instead
of stopping it itself. This is preferable, because other messages sent
to the actor that is not processed yet, may now send data to the client.

E.g. it makes this work:
$ echo status | nc localhost 6600
Add extra zeroconf stuff in http to announce mopidy-http as it's own service

This is intended to help clients using the websocket api discover mopidy
Merge branch 'develop' into feature/zeroconf-announce-mopidy
Update default http config to add new zeroconf-websocket option
Update tests to add zeroconf-websocket config option
local: Ensure directory for library exists (fixes #703)
docs: Use badges from Shields.io
travis: Use 2.7_with_system_site_packages so tox with sitepackages=true can find gst/glib/etc
Merge pull request #722 from trygveaa/develop

network: Respond to messages before closing connections
docs: Update changelog
Merge trygveaa/fix/bug-703-library-ensure-directory-exists into develop
local: Created needed dirs when scanning (fix #703)
local: Add missing file
Merge wlcx/feature/zeroconf-announce-mopidy into develop
http: Polish Mopidy-HTTP Zeroconf service

Builds upon #725 to:

- Remove extra config for Mopidy-HTTP Zeroconf service
- Refactor Zeroconf setup code in the HTTP frontend a bit
- Add documentation