Issue#83

This pull request should address the refactoring for allowing the implementation to rank and select the mfa method to branch to by comparing the current requested ranked one (the current mfa transaction) with any previously fulfilled mfa authentications for the current CAS SSO session.

The only user-facing configuration change is the wiring of the existing mfa ranking configuration Map into the `OrderedMfaMethodRankingStrategy` bean like so:

```xml
<bean id="authenticationMethodRankingStrategy" class="net.unicon.cas.mfa.authentication.OrderedMfaMethodRankingStrategy"
          c:mfaRankingConfig-ref="supportedAuthenticationMethodsConfig"/>
```
Does it make sense to delegate this to `DefaultAuthenticationMethodVerifier`?