Made sipsimple.core.Route properly subclassable

darcs-hash:20090416155748-58103-6a37b54592666715d4d38c77e9acb28f641adb46
Made sipsimple.core.Credentials properly subclassable

darcs-hash:20090416160111-58103-239226dc7a6ba946401448f1a73011a8fffdd00a