[4.1] Added access to GraphList meta data

Apparently I'm bored... or it's just PR-Friday and I'm catching PR-fever... :)

As pointed out by Christopher Thomas in a recent [comment on my blog](https://www.sammyk.me/facebook-sdk-v4-for-php-what-you-need-to-know#comment-1722399526), there is some meta data returned by Graph that cannot be accessed by the SDK.

This PR gives direct access to the meta data as well as adds better support for working with cursors when paginating.

It also provides a helper method `GraphList::getTotalCount()` for grabbing the `$.summary.total_count` value when the `summary=true` modifier was sent with a request.

I updated the docs to explain it in more detail. :)