code: function Signature( options ) {
    
    if( !(this instanceof Signature) )
      return new Signature( options )
    
    // Signing algorithm
    this.algorithm     = 'rsa-sha256'
    // Headers canonicalization / normalization type
    this.canonical     = 'simple'
    this.copiedHeaders = []
    // Signing domain
    this.domain        = null
    this.expires       = null
    this.hash          = null
    // Headers
    this.headers       = []
    this.identity      = null
    this.length        = null
    this.query         = 'dns/txt'
    this.selector      = null
    // DKIM Signature data (base64)
    this.signature     = null
    this.timestamp     = null
    // DKIM version
    this.version       = '1'
    
    var self = this
    
    if( options != null ) {
      Object.keys( options ).forEach( function( k, v ) {
        if( Signature.fields.indexOf( k ) !== -1 ) {
          self[ k ] = options[ k ]
        }
      })
    }
    
  }
docs: Signature Constructor
  @return {Signature}