Preconfigured "secret" token

[stbuehler]

Juvia::Application.config.secret_token is supposed to be a secret token; it has to be generated for each instance configured.

Right now probably most people running juvia will use the "secret" published in this repo.

The secret should be set in a config file; in production/development it should not start without a secret, test should probably have its own default secret.

[stbuehler]

Proposal to read from ENV, application.yml and then generating secret on the fly: stbuehler@20f8bce