Requesting Fields in Profile

Quick and dirty implementation for `profileFields`, as discussed in #9.

Here, `profileFields` is a parameter of strategy, and not of `authenticate`/`authorize` methods. Also, it is not passed around from method to method, but stored in an instance of the strategy. I'm not sure if this plays well with your coding conventions, but this sure was the simplest way to get the job done. Otherwise, this change would have required fixing the oauth module, too - to make it pass options to `userProfile` method and such.

I'd love to hear your opinion before I start hacking the twitter and other providers' strategies. :)