Removes Repost model.

The ability to embed an Activity exists within the Feed. Original feeds
are denoted by the feed_id field in the original Activity.
Adds Feed discovery method to model.

This will allow a Feed instance to be persisted given the feed location.
Adds discover method to Activity.

Uses Lotus to discover an external Activity if it can and add it to the
database.
Adds discovery method to Identity.

This method will use Lotus to discover the identity and associated
structures when given an account.
Adds discover method to Author.

This method will use Lotus to discover the author of a feed or when
given a user account and will store it in the database.
Also discovers Identity when Author is discovered.
Create the Feed as well when discovering an Author.
Adds db/auth dependencies.