[Authentication] Move GetSecrets, SetSecrets and related

[Stef Walter]

Some changes, patch included.

 * I've implemented GetSecret, SetSecret on the item interface.
 * The Secret struct has an object path to the session used
   to encode it instead of a the algorithm (which was rather
   useless as Michael pointed out).
 * Added secret parameter to CreateItem so it's not a two
   step process.
 * Move GetSecrets to Service interface.
 * Updated documentation.

Unless there are objections, I'll be committing this change. Thanks for
looking it over.

Cheers,

Stef