Fixes bug 1087004 - Added duration to events in Roku channel feed

To prove the point I made above about `duration` being a NULLable field. Do something like this:
```python
...
# the default is 1 hour
ok_('<runtime>3600</runtime>' in response.content)

event.duration = 12
event.save()
response = self.client.get(url)
# no longer the default
ok_('<runtime>12</runtime>' in response.content)
```