Add X509 Alternative Names resolver

Hi, 

I have written an X.509 Alternative names resolver. Besides others it allows to read Email addresses from the certificates and provide them as user names. This is especially useful for lightweight integration with Liferay in case there is no LDAP access to the certificate authority availlable and Liferay authenticates users according to their Email address.

It is a fork from the X509CertificateCredentialsToSubjectPrinciplalResolver	class.

I'm using it with

<bean class="org.jasig.cas.adaptors.x509.authentication.principal.X509CertificateCredentialsToAlternativeNamesPrincipalResolver" p:descriptor="$RFC822NAME" />

In my project for which I developed it. It would be nice if you could include this feature in future CAS versions.