Remove unnecessary parameter in _update_fields
Add flake8 config
Deserialization proof of concept
Only save underlying exception if it is an Exception
Add _WrappingException and Deserialization error to docs
Add Boolean field deserialization
Add test stubs for field deserialization
Bump dev version
Implement deserialization for DateTime, Fixed, and Price fields

DateTime implementation is incomplete (doesn't handle timezones)
Also add test stubs
Add ForcedError
Large refactor of the Fields interface

- Concrete classes define _serialize and _deserialize.
- Remove @validated decorator. Validation occurs with the within the
  output and deserialize methods of a field.
Update Field interface
Rename Field#format -> Field#_format
Update changelog
Update docs
Merge branch 'refactor' into deserialization
Document _format, _serialize, and _deserialize

Also implement Enum field deserialization
Rename CHECK_NONE -> _CHECK_REQUIRED

Also implement Email field deserialization
Test reorganization
Remove unnecessary hasattr and getattr