Fix Nested doc
Document Field#outut params
Add Serializer.factory function
Update changelog
Update dev version
Save underlying_exception as an instance variable of a MarshallingError
Document factory method
Fix indentation
Test against py34 on travis
Add fields.__all__

This will make it possible for 3rd-party libraries to extend the fields
module
Add Enum alias
Expose OPTIONS_CLASS as a class variable
Add Serializer#process_data
Bump version and update changelog