by aangel, Dave Cohen: moved fb_user_app table, which tracks users who have authorized apps, to its own module.

by aangel, Dave Cohen: stop using authmap table, use new table fb_user instead, for mapping local accounts to facebook ids.

numerous changes, especially to session management and url rewriting. performance improvements and code simplification.

Added a facebook user id column to the fb_user_app table.  This duplicates information in the authmap table, but is more efficient to query this way.

Re-arranged a number of things to make it possible to connect to facebook API from non-canvas pages.  Added fb_actions, where actions will live that invoke facebook APIs.