Entry.links

[lemon24]

We should expose Entry.links (Atom, feedparser), so it can be extended by plugins like reader._plugins.share.

Some thoughts:

• Enclosures are a kind of link (at least for Atom). Should Entry.enclosures become a property? What about the reader.enclosure_dedupe plugin that updates .enclosures? Would this be a backwards-compatible change?
• Entry.link should likely be a property returning the link with rel=alternate. Is this what feedparser is doing, or is there additional logic?
• What should the rel of custom link types like the ones coming from the share plugin be? Atom says rel can be any fully qualified URI. (Would reader:share/twitter work?)
• Idem for enclosure tags (although alternate is probably OK here).

[lemon24]

Note: Now we're storing the entry source (#276); if we're adding feed.links, we should add entry.source.links too.