using chai.js

@kenhkan this fixes BrowserStack timeout issue, now we will get for pixbi/sdk:

```
Yorkies-Air-2:sdk yorkie$ browserstack-runner --verbose
Using config: /Users/yorkie/codebase/pixbi/sdk/browserstack.json
Launching server on port: 8888
[Thu Dec 18 2014 15:26:12 GMT+0800 (CST)] Launching tunnel
[Thu Dec 18 2014 15:26:16 GMT+0800 (CST)] Tunnel launched
Launching 1 worker(s) for 1 run(s).
[OS X Mountain Lion, Chrome latest] Finding version.
[OS X Mountain Lion, Chrome latest] Version is 39.0.
[OS X Mountain Lion, Chrome 39.0] Launching
[OS X Mountain Lion, Chrome 39.0] Launched
[OS X Mountain Lion, Chrome 39.0] Acknowledged
[OS X Mountain Lion, Chrome 39.0] Tracebacks:
{ name: 'AssertionError',
  message: 'expected \'token string\' to equal \'an email\'',
  showDiff: true,
  actual: 'token string',
  expected: 'an email',
  stack: 'AssertionError: expected \'token string\' to equal \'an email\'\n    at Context.<anonymous> (http://localhost:8888/public/index.js:1746:46)\n    at callFn (http://localhost:8888/public/vendor/mocha.js:4428:21)\n    at Test.Runnable.run (http://localhost:8888/public/vendor/mocha.js:4421:7)\n    at Runner.runTest (http://localhost:8888/public/vendor/mocha.js:4823:10)\n    at http://localhost:8888/public/vendor/mocha.js:4901:12\n    at next (http://localhost:8888/public/vendor/mocha.js:4748:14)\n    at http://localhost:8888/public/vendor/mocha.js:4758:7\n    at next (http://localhost:8888/public/vendor/mocha.js:4696:23)\n    at http://localhost:8888/public/vendor/mocha.js:4725:5\n    at timeslice (http://localhost:8888/public/vendor/mocha.js:5989:27)' }
{}
[OS X Mountain Lion, Chrome 39.0] Failed: 50 tests, 48 passed, 2 failed; ran for 307ms
[OS X Mountain Lion, Chrome 39.0] Screenshot: https://s3.amazonaws.com/testautomation/a2032771cf575dd1d6b1f1fb6c7e277b6bf4a662/js-screenshot-1418887595.png
[OS X Mountain Lion, Chrome 39.0] Terminated
All tests done, failures: 2.
Exiting
```