Simple-workflow

- 942a2a3 Update models.history.builder: add History.add_activity_task_timed_out()

- 5b3142a Add module swf.models.history.builder with the History class

- Fix WorkflowExecution.signal(): fix empty input (regression introduced by previous release)

- Fix error handling on unknown resources: distinguish the type of resource.
- Add decorators to explicitly show how exceptions are handled.

- no longer fix `boto`'s version to install the latest package

Minor fixes:
- fix decisions with a action that contains an underscore
- fix actors.deciders: import missing exceptions
- fix tests by removing test_decider