ext: Add library updaters to extensionss
local: Switch to using updater from extensions
local: Remove updater from backends
backends: Add change track helper to playback provider
utils: Switch to urlparse for file-uri/path handling
utils: Convert path_to_uri to single argument
local: Convert mopidy-local extension to local: uris.

- Use local:playlist:<name>, local:track:<path> and local:search uris
- Adds LocalPlaybackProvider which translates to file uris.
- Switches to storing actual uris in playlists - so local: urls and not
  file:// or plain paths.
- Moved file:// to streaming plugin
- Cleaned up tests and imports for these changes.
Strip invalid characters from playlist names for MPD frontend

Fixes mopidy/mopidy$480 and mopidy/mopidy#474
local: Review fixes.
Change docs license to Apache License, Version 2.0

I've looked through all contributions to the docs. Any contributions not by
Thomas Adamcik or myself are trivial (e.g. typo fixes) or have mostly been
replaced by later changes. Thus I'm comfortable with changing the license with
the agreement of Thomas Adamcik.

By changing the documentation license to be the same as the source code
license, we avoid ambiguities around the license of e.g. docstrings which are
written as part of the source code but are included in the generated docs.
Scanner: Fix deadlock on incorrectly identified files
audio: Add visualiser support and adjust playbin flags

Sets up playbin to use just audio, soft_volume and downloading.
audio: Fix tests with respect to visualizer support
audio: Update changelog and docs for visualizer.
Throw an error when scan_timeout config is not loaded.
typo in regex
Merge pull request #481 from AlexandreP2101/develop

Improve proxy support by adding port and scheme to config.
docs: Update changelog, authors
Match MPD implementation and add tests
docs: Fix formatting of code examples in lists