Fix ConnectedSWFObject: restrict attributes set by constructor

- credentials: SETTINGS | kwargs
- region: SETTINGS | kwargs | boto.swf.layer1.Layer1.DefaultRegionName
- connection: kwargs
Fix ModelDiff: add missing operators and instanciate ModelDiff.container as an OrderedDict()

- Add:
  - __len__
  - __getitem__: returns a Difference(attr, local, upstream) object
- ModelDiff.container is now an OrderedDict instead of a dict
- fix tests:
  - compare len() of empty ModelDiff instead of []
  - use right attribute names for Difference objects
Fix tests.models.test_event: fix calls to constructor and len()