Added stored token secret as default value in OAuth1.0 access token requests

As in both OAuth1.0 examples (i.e. Twitter and Fitbit) the request token secret is extracted manually from the storage, I thought it could be used as a default value of `requestAccessToken(...)`.
This way the `TokenStorageInterface` has only to be handed over to the `ServiceFactory` and no manual interaction with the storage is needed.

If needed, I will change the examples accordingly.