Adds info_fields option

I'm currently working on a project where we wanted to get the `age_range` of a user without having to access the API again. This is available via the same `/me` endpoint that users' info is currently being populated with, it simply requires the presence of a `fields` parameter set to a comma-separated list of field names. To solve this problem we added an `info_fields` option to the strategy which allows developers to specify exactly which information is returned:

```ruby
provider :facebook, FB_KEY, FB_SECRET, info_fields: "first_name,age_range"
```

The same option has the added advantage of allowing developers to limit the amount of information returned for performance reasons.

If you like the way I've set this up I'll go ahead and write some tests for it.