AsakusaSatellite::Application.config.secret_token is insecure

We should keep ```secret_token``` value secret in config/initializers/secret_token.rb.
But AsakusaSatellite keeps the value as plain.

refs:
http://daniel.fone.net.nz/blog/2013/05/20/a-better-way-to-manage-the-rails-secret-token/