forms: add empty `exclude` list to OccurrenceForm.

Avoid the following warning when using Django 1.7:

```
RemovedInDjango18Warning: Creating a ModelForm without either the 'fields' attribute or the 'exclude' attribute is deprecated - form OccurrenceForm needs updating
  class OccurrenceForm(forms.ModelForm):
```
Merge pull request #22 from micrypt/patch-2

forms: add empty `exclude` list to OccurrenceForm.