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
Make sessions optional

- Introduce `option.session` so one can turn off sessions
- If you have auth schemes which involve more than one 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.
Merge pull request #1 from pvorb/patch-1

Make sessions optional
Merge pull request #7 from vanthome/patch-1

Update flatiron-passport.js