Addeding .only() to HYDReflectiveMapper. Refactoring.

- Converted HYDError codes into an enum.
- Refactored HYDSnakeToCamelCaseValueTransformer into HYDCamelToSnakeCaseValueTransformer
- Added HYDMapReflectively().only() to whitelist properties to map.
- Improved error output for debuggability.
Updated documentation based on changed in previous commit