docs: function ensureInstall(options, callback) {
    if( arguments.length === 1 && typeof options === 'function' ) callback = options;
    if( typeof callback !== 'function' ) throw new TypeError('illegal callback:' + callback);
    
    options = options || {};
    var version = options.version = options.version || '3.0.15';
    var dir = options.path = options.path || path.resolve(os.homedir(), '.mongolauncher', 'mongodb');
    var url = geturl(options.version);
    var filename = url.substring(url.lastIndexOf('/') + 1);
    var dest = path.resolve(dir, filename);
    
    fse.ensureDirSync(dir);
    
    if( !fs.existsSync(dest) ) {
      new Download({ extract: true, strip: 1, mode: '755' })
          .get(url)
          .dest(dest)
        .use(function(instance, url) {
          process.stdout.write(chalk.green('Download\n'));
        })
        .use(progress())
        .run(function (err, files, stream) {
          if( err ) return callback(err);
          callback(null, {
            home: dest,
            bin: path.resolve(dest, 'bin')
          });
        }
      );
    } else {
      callback(null, {
        dir: dest,
        version: version,
        bin: path.resolve(dest, 'bin')
      });
    }
  }