Fix BROWSERID_LOGIN_CALLBACK config setting.

With the following configuration, flask-browserid does not work.
```
app = Flask(__name__)
app.config.update(
    # ...
    BROWSERID_LOGIN_CALLBACK=browserid_callback
)
login_manager = LoginManager(app)
login_manager.user_loader(lm_callback)
browser_id = BrowserID(app)
```

The documentation said that we need to call `browser_id.user_loader(callback)` to set the callback. So it strange that when we use BROWSERID_LOGIN_CALLBACK setting, we replace user_loader method.