It should not be possible to create time frames with min=max

As long as we don't see a use case for time frames that contain only one point in time, we should not allow to create them, as the are very annoying to work with. The occur when doing intersection operations like

``` ruby
time_frame & time_frame.shift_by(time_frame.duration)
```

I would prefer to return `TimeFrame::EMPTY` here.
