Added setaccesstoken method

Hi there! Great package - really good job. :+1: 

This is my first pull request to the open source community - now you're warned! :)

I use your package through https://github.com/artdarek/oauth-4-laravel and have had some troubles getting started.

The issues is finding out a way to manually setting the access_token for the client (in my case Facebook). I am developing an API for an iOS app. The iOS app will send a facebook access token to my API, and I'll have to attach it manually.

This is my way of doing it. If you have another preferred way, that I'd not know of, please let me know :))