Be usable on Node.js

I am also using the `x18n` for the Web application.

However, during development, I sometimes run a part of application's tests by `mocha` command.
 For that reason, I changed to be usable on Node.js.

Could you add support for Node.js?


### Test results

By `mocha` command:
```
$ npm install
$ grunt
$ ./node_modules/mocha/bin/mocha spec/spec.js

  ․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․

  30 passing (29 ms)
```

And browser test is passing too:
```
$ grunt test                                 
Running "mocha:all" (mocha) task
Testing index.html....................................OK
>> 36 assertions passed (0.22s)

Done, without errors.
```