CASC-229 Make Front Channel SSO Optional so that backwards compatibility is maintained.

Problem: Spring Security adopters will fail to be able to drop in a new version of CAS Client for Java if this feature is enabled by default/required.
Solution: Force a fail-safe optional mode if the value is not provided.
Merge pull request #79 from battags/CASC-229

CASC-229 Make Front Channel SSO Optional so that backwards compatibility...