Add Post.blogged scope.

The idea of this scope is to include all posts that should be rendered in the user's public blog and its corresponding ATOM feed. This is mostly in preparation for the multitude (?) of different post types we'll have in the future.
Make PostFetcher delete posts if they've gone away (4xx)
When adding or removing a post, touch its referenced post if available.
Add Ping#source_guid and Ping#target_guid
Andy is finally an admin!
Rename 'Add Friend' to 'Follow'
Don't require presence of source_guid and target_guid in Pings
Replace Post#referenced_by array with attribute sourcing references from received pings.
When polling, deal with exceptions for each individual post that is being imported.
In /posts.atom, include a link to referenced posts.
Move hashtag autolinking into Formatter.
Add support for @mentions.
Fix mention regex to respect word boundaries.
Make #hashtag and @mention parsing a lot smarter with Nokogiri.
Gracefully deal with Markdown/etc. rendering errors.
Replace occurrences of #without_http with #to_guid
No more automatic webmentions to followings for new posts.
Retire slug and domain from post JSON.
Also link #hashtags/@mentions within HTML headings.
Fix #hashtag/@mention code to use blacklisting, not whitelisting