RSSAgent: Include `url` in addition to `urls` in each event.

An Atom/RSS entry usually has just one URL, and even if it has many only
the first of those is likely to be of interest.

Emitting an event with `url` also helps downstream agents like
WebsiteAgent that uses the key name to get a URL to crawl.
Merge pull request #594 from knu/rss_agent-url

RSSAgent: Include `url` in addition to `urls` in each event.