added login form

Login form pulls username and password from application config (not
database) and authenticates. Successful login sets a session variable
(logged in).
logout added

Gotta be able to de-authenticate
add login_required decorator and redirection

Add a "login_required" decorator to force users to be logged in to
access certain functionality.

Add a base class for the login form called "redirecting form" that
will redirect you back to the page you were interested in before getting
forced to the login page.
Title is link to index

The title in the layout is now a link to get back to the index page.
cancel edit link

added a link to the top of the edit recipe page to go back to the view
page for that recipe without making any changes.