function Signature(options, enc) {
  if (options instanceof Signature)
    return options;

  if (this._importDER(options, enc))
    return;

  assert(options.r && options.s, 'Signature without r or s');
  this.r = new BN(options.r, 16);
  this.s = new BN(options.s, 16);
  if (options.recoveryParam === undefined)
    this.recoveryParam = null;
  else
    this.recoveryParam = options.recoveryParam;
}
Returns a `Boolean` on whether or not the a `String` starts with '0x'
@param {String} str the string input value
@return {Boolean} a boolean if it is or is not hex prefixed
@throws if the str input is not a string