docs: Update name/description of Mopidy MusicBox Webclient
js: Release v0.4.0
mpd: Create unique names for all items in browse

Replace the playlist uri/name mapping with a mapping for all the items
in browse. Check the mapping when browsing a path.

This has the added benefit of serving as a cache for browse, so we don't
need to traverse the path parts and lookup each of them for each call to
browse.
http: Fix missing placeholder in log statement

Related to martijnboland/moped#5
Add Mopidy-Spotify-Tunigo to list of extensions
Merge pull request #764 from trygveaa/docs/add-extension-mopidy-spotify-tunigo

Add Mopidy-Spotify-Tunigo to list of extensions
Merge pull request #762 from trygveaa/fix/mpd-browse-duplicate

mpd: Create unique names for all items in browse
docs: Formatting
docs: Add Mopidy-API-Explorer
docs: Add PR#762 to changelog