Integrate Action Pack with Rack 1.5

All ActionPack and Railties tests are passing.

[Carlos Antonio da Silva + Santiago Pastorino]
Remove Session::Options in favor of setting the options into env directly

[Carlos Antonio da Silva + Santiago Pastorino]
Session now inherits from Rack::Session::Abstract::SessionHash

[Carlos Antonio da Silva + Santiago Pastorino]
Leave Session.create responsibility to the SessionStore

[Carlos Antonio da Silva + Santiago Pastorino]