fix crash when show About dialog

fix crash when show About dialog

```
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/hamster/today.py", line 340, in on_menu_about_activate
    dialogs.about.show(self.window)
  File "/usr/lib/python2.7/site-packages/hamster/configuration.py", line 111, in show
    self.dialog_close_handlers[dialog] = dialog.connect("on-close", self.on_close_window)
AttributeError: 'About' object has no attribute 'connect'
```