(PE-5344) DRYed out authority-key-identifier Clojure function

This commit extracts some of the redundant map setup in the different
arities of the `authority-key-identifier` function into a helper
function, `create-authority-key-identifier`.  The helper function takes
advantage of the fact that the underlying Java method that interprets
the map was already able to handle the absence of a key in the map vs.
the presence of the key but with a null value as being equivalent.
Merge pull request #30 from camlow325/feature/master/PE-5344-add-extensions-for-master-ca-crl

(PE-5344) Cert extension changes to support generation of Master, CA, and CRL certs
(PE-5395) Add extensions support to generate-crl

This commit adds the ability to pass in a list of extensions to the
generate-crl Clojure function and CertificateAuthority.generateCRL()
Java method.

This commit also adds the ability for the Clojure get-extension*
functions to handle getting extensions from an X509CRL.  This is
facilitated via new underlying getExtension*() methods on the
ExtensionUtils Java class.  Support for extraction of extensions
from a CRL was added.