Issue #73, changes DataProvider#loadSession to return the session.

DataProvider#loadSession now returns the session instead of setting
up the req.session helper itself. This permits moving the logic
for setting up req.session to the session middleware simplifying
other implementations of session data providers.
Whitespace
renames dataProvider to cookieDataProvider and corrects save call to use req.env.session without the sessionId qualifier
Change to match coding conventions
Moves session middleware beneath the middleware directory.