Example "auth" is not working

```
*.is_authenticated()
```

throwing:

```
File "/tmp/flask-admin/examples/auth/app.py", line 105, in index
    if not login.current_user.is_authenticated():
TypeError: 'bool' object is not callable
```

Fix:
Change **all** is_authenticated() to is_authenticated (in all *.py and *.html)

€dit: Tested with python2.7, 3.4 and 3.5
