Refactor to remove import of core.py in fields.py
Add class Meta with fields option
Refactor to remove import of core.py in fields.py

Also rename core.py -> serializer.py
Merge branch 'meta' into dev

Conflicts:
	marshmallow/__init__.py
	marshmallow/core.py
	marshmallow/utils.py
	tests/test_marshmallow.py
More useful error msg when declaring field as class
Add exclude option to class Meta
Allow changing of date format

by passing format to DateTime constructor
Map both string types
Fix nested serializers with Meta fields
Add failing tests for nested fields with Meta fields option