function tower() {
  var args = Array.prototype.slice.call(arguments);
  var fn = args.pop();

  args.push('--output-directory', 'tmp');

  var child = spawn('./bin/tower', args);
  var result = '';
  var error = '';

  child.stdout.setEncoding('utf-8');
  child.stdout.on('data', function(data){
    result += data;
  });

  child.stderr.setEncoding('utf-8');
  child.stderr.on('data', function(data){
    error += data;
  });

  child.on('close', function(){
    fn(error ? error : null, result);
  });
}
Execute a tower command, return output as string.