CAS-1411: Update protocol spec with details on proxy callback behavior

The commit updates the protocol to provide more implementation detail as for the conditions that need to be passed and verified for a successful proxy authentication scenario. Emphasis is on failure scenarios. Furthermore, changed the recommendation on the format of attributes that are extended in Appendix A to match the default behvior.
Merge pull request #387 from Unicon/CAS-1411

CAS-1411: Update protocol spec with details on proxy callback behavior