Update flatiron-passport.js

Hi, please have a look a this PR.
It enhances 2 things:
1.) If you don't use sessions, you can now turn it off by 
app.use(fipassport, {session: false});

2.) If you have auth schemes which involve more than on handler like
an OAuth token endpoint for example, you need to consider the callback of flatiron 
and call it accordingly; otherwise only the first handler will get called.

pls. accept if you like.

Thanks,
Thomas