Update swf.actors.Decider.poll() #24: return the full history (all pages)

It introduces the cumulated latency of the API call for each pages
(`poll_for_decision_task()`). All pages are merged in a single History
object.
Add tests for Decider.poll()
Merge pull request #25 from botify-labs/feature/decider-poll-full-history

Feature/decider poll full history