Write the public key to the meta data (#21)

Signed-off-by: Nico Rutishauser <nr@hotmail.ch>
Read the cached public keys (#21)

Signed-off-by: Nico Rutishauser <nr@hotmail.ch>
More tolerant when no persisitence meta file exists

Signed-off-by: Nico Rutishauser <nr@hotmail.ch>