add authentication error constants

C. Fahner <info@fahnerit.com>
add require authentication function

C. Fahner <info@fahnerit.com>
add requireAuthentication shorthand

C. Fahner <info@fahnerit.com>
add getCurrentId shorthand to Service

C. Fahner <info@fahnerit.com>
add optional user reference to storeSnapshot

C. Fahner <info@fahnerit.com>