Support paging in Decider.poll()

[ggreg]

When a decider polls a workflow execution, SWF returns a token and the workflow's history. The history contains at most maximum_page_size events where maximum_page_size cannot be greater than 100. It means a history with more than 100 events is scattered through several pages (len(history) / maximum_page_size pages).

The purpose of this issue is to abstract SWF's maximum_page_size by:

• supporting abitrary page sizes (even greater than 100) in Decider.poll()
• by default (size=None) retrieve all pages

[ggreg]

Returning the whole history is enough.