code: function pad (n, width, z) {
    if (typeof n !== 'string') throw new Error('bitString#pad: Need String');
    z = z || '0';
    while(n.length < width) {
      n = z + n;
    }
    return n;
  }
docs: Pad a string to atleast desired width
  @param  {string} n      String to pad
  @param  {integer} width Width to pad to
  @param  {char=} z       Optional pad char, defaults to '0'
  @return {string}        Padded string