Added refresh token

[tscheepers]

As stated in the documentation by @moneybird, the refresh token should be persisted in all cases. This is not possible with the current implementation.

The response contains an Access token which you can use to connect with the API. The Refresh token can be used to retreive a new access token in case the access token can expire. In that case, an expires_in is given. Both the access token and the refresh token should be persisted to be used for future requests.

It's a bit unclear to me whether the access token can actually expire, since I was not able to retrieve an expires_in. In any case we should be able to persist the refresh just in case.

Another small change here is making the getAuthUrl public since we then could use framework methods for redirecting, e.g. in Laravel: return redirect($connection->getAuthUrl());.