Support Unicode paths for ftp transport (encoded as utf8).

* bzrlib/transport/ftp/__init__.py:
(FtpTransport._remote_path): Remove the work around, we send and
receive utf8 paths.

* bzrlib/tests/per_transport.py:
(TransportTests.test_unicode_paths): Check that the *server* can
support Unicode paths).

* bzrlib/tests/ftp_server/pyftpdlib_based.py:
(BzrConformingFS.ftp2fs): Added to decode utf8 received paths.

* bzrlib/tests/ftp_server/medusa_based.py:
(FTPTestServer): Declare that medusa can't support Unicode paths
in an had-hoc way.

* bzrlib/osutils.py:
(fancy_rename): Ensure all paths are unicode.