Add support for generate_session_secret

[oliland]

I use pyfacebook as a session proxy for iPhone applications using Facebook Connect. In order for Facebook to return a session secret, you must pass "generate_session_secret=1" as a GET variable in the getSession() method.

This is easily patched by adding generate_session_secret as a named paramater in init() and adding generate_session_secret to args{} in auth.getSession()