replace casperjs with selenium

It's too verbose to assert the return code is 200 for every test request. A better way is like this:

```python

# base.py
class SeahubApiTestCase:
    def get(self, url, expected=200):
        res = self.requests.get(url)
        self.assertEqual(res.status_code, 200)
        return res

# test_accounts.py
class AccountsApiTestCase(SeahubApiTestCase):
    def test_some_account_api(self):
        res = self.get(SOME_URL)
        ...
```