Added simply logging

Hello!

Debugging can be very hard with your current implementation.
I've added an simple logger instance which logs the traceback on an Exception.

Output is like:

```
[24/Aug/2014 20:18:48] ERROR [django_ajax.decorators:inner():75] list index out of range with json <QueryDict: {u'action': [u'1'], u'caller': [u'2'], u'data': [u'{"username": "xxxxx", "password": "xxxxx"}']}>
[24/Aug/2014 20:18:48] ERROR [django_ajax.decorators:inner():76]   File "/usr/lib/python2.7/site-packages/django_ajax/decorators.py", line 72, in inner
    return render_to_json(func(request, *args, **kwargs))

  File "/var/gui/interface/views.py", line 70, in api_request
    api.account_actions(request, action, data, values)

  File "/usr/lib/python2.7/site-packages/django/contrib/auth/decorators.py", line 22, in _wrapped_view
    return view_func(request, *args, **kwargs)

  File "/var/gui/interface/utils.py", line 54, in account_actions
    return account_views.settings_get(request, values)

  File "/usr/lib/python2.7/site-packages/django/contrib/auth/decorators.py", line 22, in _wrapped_view
    return view_func(request, *args, **kwargs)

  File "/var/gui/account/views.py", line 161, in settings_get
    settings = settings[2]

  File "/usr/lib/python2.7/site-packages/django/db/models/query.py", line 177, in __getitem__
    return list(qs)[0]
```

Greets
Manuel