The filesystem storage backend now implements the required functions to
act as a basic CalDAV server. Some refactoring was done based on the
go-webdav development: introduce a UserPrincipalBackend, a new function
to serve the user principal URL, and more. See this PR for lots of
details: https://github.com/emersion/go-webdav/pull/62
Also adds a simple facility for debug output.
