redis-backed "mentions" timeline as redundant (given notifications)

(without avatar). Also improved search position of exact matches

Only distribute statuses to followers who signed in in the last 2 weeks, add rake task for clearing feeds of inactive users

to the API

respect hub.lease_seconds (fix #46)

and atom feed generation to some extent, as well as the way mentions are
stored

local followers. Remote users' usernames SHOULD be case-sensitive