Modify the CAS server to be able to remember the authentication method used.

Appropriately modify the CAS protocol to return the authentication method in the validation payload, or decorate correct error messages based on the requested method.
Modify the mfa authentication request to be able to conditionally require mfa.

Added additional tests in support of new behavior.
Added authentication method along with user attributes to the validation xml response.
trivial: request --> requested in JavaDoc
Remove rebel.xml from the codebase.
Rename ServiceValidateController. Address code reviews.
Merge branch 'REMEMBER_AUTHN_METHOD' of github.com:Unicon/cas-mfa into REMEMBER_AUTHN_METHOD
Fix test failures on the spec validation.
Fix a typo, and correct the javadoc in a few spots.
Document how to build cas-mfa-java
Documents CAS server GH issue requesting non-final
Documents with URL the CAS server GitHub issue requesting non-final SimpleWebApplicationServiceImpl which would simplify this class.
trivial: "can be retrieve" --> "can be retrieved"
clarifies JavaDoc of Validate...MFARequestAction
partial progress draft documenting architecture as coded
zapped link to gone GitHub issue
Jasig/CAS GitHub issue tracker retired, so link to GitHub issue re un-final-marking the wrapped class no longer works.  Removed link from JavaDoc.
clarified JavaDoc re handling of no-existing-SSO-session case
Stop unnecessarily considering request parameters.
remove unnecessary mocking of request parameter map un-empty-ness
remove mocking of request parameters
Request parameters are parsed by argument extractor, not by Action.  They are irrelevant to the Action.
trivial: isNotBlank() rather than !isBlank()