Add a check to determine if the JCE jars are available

[ashmeer7]

This would be helpful for people for debugging their setup.

A warning that they don't have the JCE unlimited strength jars might be enough.

[bluekeyes]

It looks like this is simpler to implement than I thought: http://stackoverflow.com/questions/7953567/checking-if-unlimited-cryptography-is-available

That said, I'm not sure where this check should be: SSH seems to generally work without these extensions, so it should probably be part of Kerberos, but could happen either when you construct a credential or when you authenticate. Currently, I'd suggest logging before authentication happens or logging after authentication, if it fails.