Extend issued-by? to support X509CRLHolder

When you deserialize a CRL as created by generate-crl, you don't get
the same CRL class back - you get a BouncyCastle X509CRLHolder.
This commit adds support for this class to the issued-by? multimethod.
"The contract of issued-by? is supposed to be that it abstracts the method of retrieving and testing against the "issuer" field on any SSL/CA-related objects."

That seems too general to me.  My vote would be to put this part of the code back how it was before (or even to reduce the number of forms of the multimethod).

If you need a utility function that is specific to CRL's in your test namespace, create it there.  And preferably include 'crl' in the name of it.

If you're concerned about having to look at javadocs to determine the names of methods available on a particular SSL related object, then maybe we should define clojure protocols for the important objects, and use schema or post-conditions to indicate which functions are returning instances of which protocols.  That seems like a much more manageable approach than just overloading all of our functions to try to make them accept any old type of input.

This is my 2 cents, anyway.  Maybe you should wait and let someone else weigh in to see if I'm in the minority again.