Fix/unknown resource handling

Fix #35

- Add decorators in `swf.exceptions`:
   - `@catch` (also aliased as `when`),
   - `@translate`
   - `@is_not`
- Add specific exceptions in `swf.models` modules `domain`, `activity`, `workflow`.
- Wrap model methods with `@when`, `@is_not`, and `@translate` to hide the plumbing and group it in the same place.
- Fix tests (unit and doctests).
- Enable doctest in travis.

I chose to decorate methods instead of patching the `boto.swf.connection` methods to avoid messing with mocking and having an implicit behaviour.