News data fixes

- Try to pass around object IDs instead of managed objects (may be a bit of overkill here)

- Fix a bug where cached stories would not be cleared after an update
Fixes an occasional crash when viewing a story

Use 'existingObjectWithID:error:' so an exception isn't thrown when the object doesn't exist (the controller just complains and shows nothing)
[#31] Tapping 'In the Media' stories should offer to open them in Safari