code: function(publicKey, callback) {
          request.head(
              this.getResourceUrl({ path: '/keys/' + publicKey, user: null }),
              get404Handler(callback)
          );
          return this;
      }
docs: Check whether a public key exists or not
  
  @param {String} publicKey Public key you want to check for the presence of
  @param {Function} callback
  @return {ImboClient}