This adds the 'airsonic.rememberMeKey' system property (can be set from
command-line with `-Dairsonic.rememberMeKey=<value>`) as well as a
'RememberMeKey' setting in airsonic.properties, so that the key used for
generating 'remember me' tokens can be persisted across server restarts.
It also adds a default, insecure key in case we are running in
development mode with the 'airsonic.development' property set.
François-Xavier Thomas
8a90d9f77b