Rearchitect filters as event listeners

[kern]

Currently, filters are callbacks which receive the full thread as an argument, but this isn't terribly smart since we have to diff against previous versions of the thread to see what's changed. This is especially problematic with edits because the Graph API doesn't give you any indication that a post's content has changed, and hence commands can be issued retroactively if you edit old comments.

I'm moving over to an evented model for representing changes to the group's feed on the sentiment branch. There will be a PostEmitter of some sort which emits new, edit, and delete events whenever a post or comment is created, edited, or deleted. Persistence will be baked in so that we can diff thread versions, probably through an embedded database like nedb.