config: Add a Path config value and an ExpandedPath wrapper.

Allows us to easily use expanded paths, without losing the original value for
display and storage. In theory we could be using same trick for passwords.
config: Switch all paths and files to use the Path in schemas.

Also renames static_dir to static_path for better consistency.
config: Add path config value tests
Merge branch 'develop' into feature/config-path

Conflicts:
	mopidy/backends/spotify/__init__.py
Merge pull request #391 from adamcik/feature/config-path

Add a Path config value that provides expanded path and original.