Allow user to pass params to passport.authenticate

This allows you to pass in parameters to a the passport.authenticate function. All you have to do is add a params key to your strategy.

Here's an example strategy, passing in a scope for facebook:
```coffeescript
# Authentication setup
strategies =
  github:
    strategy: require("passport-github").Strategy
    conf:
      clientID: process.env.GITHUB_CLIENT_KEY
      clientSecret: process.env.GITHUB_CLIENT_SECRET
  facebook:
    strategy: require("passport-facebook").Strategy
    conf:
      clientID: process.env.FACEBOOK_KEY
      clientSecret: process.env.FACEBOOK_SECRET
    params:
      scope: ['read_stream', 'publish_actions']
```

See *permissions* on http://passportjs.org/guide/facebook/.

This fixes https://github.com/lefnire/derby-auth/issues/14.

EDIT: Fixed example.