added property Card.date_last_activity : datetime, and test for fetching card attributes (including the new datetime property)
Unified initialization of cards from JSON data. Added optionally lazy properties for comments and checklists.
Python 2 fix (@henriquegemignani )
version bumped and dependency "dateutil" added
Merge branch 'master' of https://github.com/gameloop/py-trello

Conflicts:
	trello/__init__.py