got basic annotation infrastructure working

	* libnautilus-extensions/Makefile.am:
	* libnautilus-extensions/nautilus-annotation.c:
	(digest_file_completed), (digest_file_failed),
	(calculate_checksum_callback), (read_file_open_callback),
	(calculate_file_digest), (get_annotation_path),
	(look_up_local_annotation), (has_local_annotation),
	(got_annotations_callback), (fetch_annotations_from_server),
	(get_annotation_from_server), (got_file_digest),
	(nautilus_annotation_has_annotation):
	* libnautilus-extensions/nautilus-metadata.h:
	* src/file-manager/fm-icon-view.c: (get_icon_text_callback):