Friendly string representation of entities

[pswaminathan]

__repr__ and __str__ should be more expressive. Like with http://stackoverflow.com/a/2626364/1819790. repr(entity) should return an unambiguous representation of the entity, perhaps in kwarg form (such as Advertiser(id=123456, name='Some advertiser', ...)). str(entity) should be a pretty form of it; perhaps pretty-printing properties