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'