function _stringToArrayBuffer(str, callback) {
    var bb = new Blob([str]);
    var f = new FileReader();
    f.onload = function(e) {
        callback(e.target.result);
    };
    f.readAsArrayBuffer(bb);
  }
Converts a string to an array buffer

@private
@param {String} str The string to convert
@param {Function} callback The function to call when conversion is complete