Include series information in RSS feeds

[lyzadanger]

User feedback suggests that it's difficult to ascertain the order of a post in a series when it's in an RSS feed (this could be especially confusing in the event that I add series ordering support that is not just chronological). It would be helpful to ensure that series info (e.g. "This post is 2 of 5 in a series about..." with a link to the series page or something) is in RSS-syndicated content.