avahi: More review comments and some other fixes.

- Switched to newstyle class
- Switched to safe values in kwargs
avahi: Remove use of config_section
Merge pull request #573 from adamcik/feature/avahi

Cleaned up version of #558 - Add avahi publishing of MPD and HTTP server endpoints.
http: Fix tests
docs: Update changelog
docs: Update authors
docs: Add {mpd,http}/zeroconf descriptions
docs: Fix typos
Merge pull request #569 from ZenithDK/scan_multiple_artist

Fix scan with multiple track artists and add tests
docs: Update changelog
Merge branch 'develop' into feature/filter_improvement
core: Change tracklist.{filter,remove} usage

The criterias are now a mapping between field names and one or more values.
This aligns tracklist.{filter,remove} with the API of
library.{find_exact,search}, and allows for e.g. batch removals.

An exception is raised immediately if the API is used in the old way to ease
migration and debugging.
Merge pull request #574 from jodal/feature/filter_improvement

tracklist.{filter,remove} with multiple criteria values
Merge branch 'develop' of https://github.com/mopidy/mopidy into feature/extra_tags
Address review comments
docs: Update changelog
commands: Start adding commands util helper.

This class is intended to be used to contruct a tree of sub-commands. In this
intial version it mainly focuses on exposing `add_argument` to allow for nested
parsers. Next step is to start adding better `--help` that shows the entire tree
and hooking it up to select what code path to really run.
commands: Set child names upon adding them
commands: Add basic format usage helper
commands: Make sure parser errors get translated to correct error type.