constructor (options) {
    options = options || {};
    this.default_key_size = parseInt(options.default_key_size) || 1024;
    this.default_public_exponent = options.default_public_exponent || '010001'; //65537 default openssl public exponent for rsa key type
    this.log = options.log || false;
    // The private and public key.
    this.key = null;
  }
@param {Object} [options = {}] - An object to customize JSEncrypt behaviour
possible parameters are:
- default_key_size        {number}  default: 1024 the key size in bit
- default_public_exponent {string}  default: '010001' the hexadecimal representation of the public exponent
- log                     {boolean} default: false whether log warn/error or not
@constructor