Rails 3.2.x compatibility

Changes:
- Updated readme
- ActionController::Session::AbstractStore is now ActionDispatch::Session::AbstractStore in rails 3
- #set_session method is now passed the rack session options as a fourth parameter
- Remove 'secret' option from docs - no longer part of rails session config (it's now a separate rails config options called secret_token)
- set_session method now returns the session's id
  - Rails 3.2.x depends on rack ~1.4.5, which requires that the session store's #set_session method return the session_id upon successful saving