a redis session store for rack. largely based on Rack::Session::Pool.
this is an initial release, use with care. patches gladly accepted.

  $ cat config.ru
  require "rack/session/redis"

  use Rack::Session::Redis, {
    :url          => "redis://localhost:6379/0",
    :namespace    => "rack:session",
    :expire_after => 600
  }

using `:expire_after` requires redis >= 2.0.