Add to_string with Integer and Float tests
Add rest of to_string tests
Use field.from_string in parse_xml
Allow option fields to be parsed by parse_xml
Export pretty-printed JSON in XML for Fields
Some refactoring
Fix PR comments
Use ddt in test_fields
Merge pull request #2 from open-craft/dragonfi/field-to-string

Adding field to_string/from_string methods that encode to JSON/YAML strings.