Allow caller to specify secret

Some users of asfquart need to maintain their own session details, and must therefore ensure that apptoken.txt is recreated in synch.

Whilst this can theoretically be done by carefully timing the deletion of apptoken.txt, it would be a lot easier if the caller could provide their own token when constructing asfquart. The caller can then use their own storage for the token and recreate it whenever they need to reset their sessions.