Setup py.test
Split test_fields into modules for listfield, dictfield, partialdictfield

* Also convert tests to py.test
fix code format issues in setup.py
Fix code format and clarify partial dict field doctstring in fields.py
Fix code formatting issues in test_in_serializers
Fix PartialDictField for validate and non-dict values

* Override validate so that non-included keys can't cause validation errors
* Factor out code to filter dict *and* protect from non-dict values (avoid blowing up on these; defer to DictField base class for handling)
Code formatting in listfield / dictfield tests
Fix ListOrItemField validation and add tests
Convert serializer tests to pytest