(PE-5344) Cert extension changes to support generation of Master, CA, and CRL certs

This PR adds support for the creation of a CRL number extension.  It 
includes a `crl-number` Clojure function which produces an extension map
which can be included as another extension in the extensions argument in
a call to `CertificateAuthority.signCertificate()`.

This PR also includes the following extensions changes:

    * Modified the mapToBasicConstraints method to support not specifying a
      value for `path_len_constraint` for a CA.  This allows the constraint to not
      define a limit for the number of intermediate certificates that may
      follow the CA in the certification path.

    * Separated the `basic-constraints` Clojure function into
      `basic-constraints-for-non-ca` and `basic-constraints-for-ca`
      functions.  This accounts for the differences in possible parameters
      for these two cases.  The critical parameter only needs to be
      specified for a non-CA because for a CA, it needs to be `true` per
      RFC-5280.  The `max-path-len` is only supported for a CA and even then
      is optional.

    * Modified the mapToAuthorityKeyIdentifier method to allow the CA serial
      number and distinguished name (issuer) to be added to the generated
      extension.

    * Split the `authority-key-identifier` function into different arities
      which would support specification of an issuer-dn, serial number,
      public key, and critical flag for the legal supported combinations.
Ah this looks strange - should just be on the line above