New interface type representing a feature that can be added to a

New interface type representing a feature that can be added to a

	* libsoup/soup-session-feature.c: New interface type representing
	a feature that can be added to a SoupSession.

	* libsoup/soup-session.c (soup_session_add_feature): Add a feature
	to the session by prepending it to priv->features and calling
	soup_session_feature_attach() on it.
	(soup_session_add_feature_by_type): Add a feature to the session
	by creating an object of the indicated type and passing it to
	soup_session_add_feature.
	(soup_session_remove_feature)
	(soup_session_remove_feature_by_type): Likewise, remove features
	(soup_session_class_init, set_property): register/handle
	construct-time feature adding/removing properties
	(dispose): cleanup features

	* libsoup/soup-logger.c: port to SoupSessionFeature

	* tests/test-utils.c (soup_test_session_new): Use
	soup_session_add_feature rather than soup_logger_attach.

svn path=/trunk/; revision=1133