Fixed references to static properties of Client (settings and client)

The client::settings object is defined in /lib/client.php as static:

    public static $settings;

But then in many places throughout the code (and api docs), it's referenced as a non-static property, like so:

    $scope = $this->settings->oauth_scope;

Instead it should be getting referenced like this:

    $scope = self::$settings->oauth_scope;

My App has php option strict turned on, so I get errors about every usage.

There was a similar problem with the $client static property too.

NOTE: You will also need to update the documentation at: https://docs.dwolla.com/?php