remove ftp/file-based interface

By now we're offering the archistar-functionality through a
S3-compatible Interface (or through REST). This should be better
suited for users trying to access archistar. In addition we can
reuse much of the existing S3 specification and do not have to
reinvent the wheel.
extract XML-answer generation into a helper of its own

next step: unify FakeBucket and FakeRoot