new and more intuitive way to edit entities

changing the item's attributes directly
and then calling .editEntity() without data

the new .toJSON() methods of WikibasePage and ItemPage also accept
a 'diffto' argument to avoid sending redundant data to the server

changed type of sources and qualifiers
from defaultdict to OrderedDict
to preserve 'snaks-order' and 'qualifiers-order'

Change-Id: Ibe77c51fc5cf3dae8ece533cc5787bf960f0ebed
Merge "new and more intuitive way to edit entities"