result: don't assume that either title or description are valid

Signed-off-by: Jeff Mahoney <jeffm@jeffm.io>
linkcache: add support for last_seen

It's useful to know when a URL was last seen, both for reporting via
RSS or for duplicate suppression.

Signed-off-by: Jeff Mahoney <jeffm@jeffm.io>
Merge branch 'master' of github.com:jeffmahoney/linkcache

Conflicts:
	linkcache/database/sql.py