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.
@camlow325 has started to voice a similar concern in that downstream projects probably shouldn't need to test (re-test, really) the functions of this library.  I agree with this, even though I only just now came around to the realization that this is what I've done.  

Still, I feel like some level of testing should be done in downstream projects, I'm just not exactly sure what that would entail if these issued-by/has-subject functions went away (or at least relegated to private test functions within this library).