Finish remote refactoring. Test suite passing.

* bzrlib/transport/remote.py:
(RemoteTransport.__init__): Simplified.
(RemoteTransport._build_medium): New abstract method.
(RemoteTransport.clone): Call the real __class__ constructor and
provide the transport to clone from.
(RemoteTransport._unparse_url): Deleted.
(RemoteTCPTransport.__init__): Deleted.
(RemoteTCPTransport._initial_split_url): New method. Set the
default port.
(RemoteTCPTransport._build_medium): New method.
(RemoteSSHTransport.__init__): Deleted.
(RemoteSSHTransport._build_medium): New method.
(RemoteHTTPTransport.__init__): Updated for cloning.
(RemoteHTTPTransport.abspath): Deleted. The inherited method is
enough.
(RemoteHTTPTransport.clone): Updated for cloning.