[Bug Fix] Fixed bug that blindly appended params to a url

I should be able to pass `FacebookRequest` a URL that already has params appended to it like so:

```php
$data = (new FacebookRequest(
    $session, 'GET', '/123/accounts/test-users?limit=10&fields=id,access_token,login_url'
  ))->execute();
```

But it will blindly append the access token and will return an ill-formed URL like this:

```
https://graph.facebook.com/v2.0/123/accounts/test-users?limit=10&fields=id,access_token,login_url?access_token=some_token
```

This PR fixes that. :)

I also fixed the doc blocks in this file.

I also made it so you can overwrite the default `access_token` via `$params`. Important for extensibility and testing. :)