Added Card.date_last_activity, unified Card initialization and added lazy properties for comments and checklists

Since I'm actively working with the library right now I wanted to add some stuff to the library.

- Added property Card.date_last_activity (type datetime) 
- Added parameter eager=True to Card.fetch to control whether comments and checklists should be fetched
- Unified card initialization logic using static method Card.from_json(json_obj, trello_list, eager=False)
- Added lazy properties for comments and checklists: If they were not fetched initially (eager=False), they are fetched on demand when the properties are accessed
- Tests extended to test new date_last_activity property and comments
- merged in the Python 2 fix of @henriquegemignani

The API and semantics have not been altered, only been added to.