Add option to pass in secret key

[Frando]

Allow to pass in a precreated key pair and store the secret key with the source feed. This allows to first create the key pair and then the hyperdb - useful e.g. if you want to manage many hyperdb and store them in dirs named after the pubkeys (otherwise you'd have to move the db after creation because you'd only know the pubkey after creation).

[mafintosh]

Great job with adding tests as well