Refactor proxy and auth test classes. Tests failing for digest auth.

* bzrlib/transport/http/_urllib2_wrappers.py:
(AbstractAuthHandler.auth_required,
AbstractAuthHandler.auth_successful): Add response parameter as an
authentication scheme may need additional info provided only
there.

* bzrlib/tests/test_http.py:
(TestAuth, TestHTTPAuth, TestProxyAuth, TestHTTPBasicAuth,
TestHTTPProxyBasicAuth): Refactoring.
(TestHTTPDigestAuth, TestHTTPProxyDigestAuth): New classes for
digest authentication tests.

* bzrlib/tests/HttpServer.py:
(TestingHTTPRequestHandler.translate_path): Handles request
proxying based on a server attribute.
(HttpServer): Add proxy_requests attribute.

* bzrlib/tests/HTTPTestUtil.py:
(ProxyServer): Replaces FakeProxyRequestHandler now that
HttpServer handles proxying based on a server attribute.
(AuthRequestHandler, BasicAuthRequestHandler, AuthServer,
HTTPAuthServer, ProxyAuthServer, HTTPBasicAuthServer,
ProxyBasicAuthServer): Refactoring Auth servers and request
handlers.
(HTTPDigestAuthServer, ProxyDigestAuthServer): New classes for
digest authentication.