Make hhtp proxy aware of AuthenticationConfig (for password).

* bzrlib/transport/http/_urllib2_wrappers.py:
(Request.__init__): Update the auth dict content documentation.
(extract_credentials, extract_authentication_uri): Deleted.
(PasswordManager): Deleted.
(ProxyHandler.set_proxy): Query AuthenticationConfig for the
password. Simplified.
(AbstractAuthHandler): Looong forgotten feedback review about
_retry_count. The behaviour is not changed.
(AbstractAuthHandler.get_password): Change signature now that the
auth dict contains all relevant information. Simplified.
(DigestAuthHandler.auth_match): Delete gratuitous differences with
BasicAuthHandler.
(Opener.__init__): Get rid of PasswordManager.

* bzrlib/transport/http/_urllib.py:
(HttpTransport_urllib._perform): Replace authuri by its components
in the auth dict.

* bzrlib/transport/ftp.py: 
Register 'aftp' for urlparse (revealed by the host empty test in
_split_url).

* bzrlib/transport/__init__.py:
(ConnectedTransport._split_url): Check that host is not empty. Was
first needed to make a proxy test pass, but revealed more bugs in
the test suite.

* bzrlib/tests/test_transport_implementations.py:
(TransportTests.test__reuse_for): Fix bug revealed by the host
empty test in _split_url.

* bzrlib/tests/test_transport.py:
(TestConnectedTransport.test_connection_sharing_propagate_credentials): 
Fix bug revealed by the host empty test in _split_url.

* bzrlib/tests/test_smart_transport.py:
(HTTPTunnellingSmokeTest._test_bulk_data): Fix bug revealed by the
host empty test in _split_url.

* bzrlib/tests/test_http.py:
(TestHttpTransportUrls.test_invalid_http_urls): ConnectionError
can't be raised here.
(TestHttpProxyWhiteBox._proxied_request): No more PasswordManager,
yeah !
(TestProxyHttpServer): Delete FIXME, tests have been added a long
time ago.

* bzrlib/config.py:
(AuthenticationConfig.get_user): Fix silly copy/paste.
(AuthenticationConfig.get_password): Fix oversight.