First implementation of transport hints.

* bzrlib/tests/test_transport_hints.py: 
New file.

* bzrlib/transport/__init__.py:
(TransportHints, TransportGetHints): New hints classes.
(Transport.create_get_hints): New method creating a suitable hints
object.

* bzrlib/tests/__init__.py:
(test_suite): Add test_transport_hints.

* bzrlib/errors.py:
(RedirectRequested.__init__): Fix typo.
(UnknownHint): New exception class.

* bzrlib/bzrdir.py:
(BzrDirFormat.probe_transport): Pass a hints object to
transport.get.