[dynamodb2] Implement nonzero for Item to consider empty items falsey

[jamesob]

Doing table.get_item(foo='a') where no item exists matching foo='a' returns an empty item. The returned empty item when passed to bool evaluates as True:

In [23]: i =  comments_table.get_item(item_id='not_in_there')

In [24]: bool(i)
Out[24]: True

This is surprising, and it'd be nice to have i show up falsey.