docs: Update mock to support glib.get_{system,user}_config_dir{s,}
docs: Move images out of the _static dir

Sphinx copies the images to _images when the docs are built. Thus the
mopidy-docs debian package contains two copies of all images; one in _static
and one in _images. By keeping the images next to the source documents we only
get the _images copy included in the Debian package, reducing the package size
by 1.5MB.
py3: Use print() function
tools: Remove unused dev tools
zeroconf: Make public API with docs
docs: Add note about public APIs and API stability
models: Add lightweight Ref model with URI, name, and type
models: Add description of Ref model
Merge pull request #621 from jodal/feature/ref-model

models: Add lightweight Ref model with URI, name, and type
Merge branch 'develop' into feature/extension-registry