Pull-request with kerberos token delegation option

Hi all,

Just wanted to spread the knowledge of a pull-request I've made with some 
nice server-side improvments on GSSAPI and Kerberos token options to use a 
credentials cache to store user's tokens. This enables CyrusSASL to not 
just be an authentication end-point, but also allows for single-sign-on to 
other services server-side.

Check it (#586) out - hope it's good-enough for the feature-set addressed.

Kind regards,
AiO