code: function replaceAt(value, index, character) {
      return value.substr(0, index) + character + value.substr(index+character.length);
  }
docs: Helper to replace characters at a specific index, js strings
  are immuatable so str[index]='' wont work.
  @param {String} value The original string to modify.
  @param {Number} index The index of the character to replace.
  @param {String} character The replacement character.
  @returns {String} The modified string.