Make the generated schema be more consistent with the serializer behaviour

[moinessim]

• Make (Non optional/nullable/skippable) record fields be required. Fixes https://github.com/panesofglass/FSharp.Data.JsonSchema/issues/9
• Do not unwrap single case unions: This allows them to roundtrip serialization and validation.
• Use NJsonSchema setting to generate documentation fields for the schema, based on F# comments (generated xml docs). Got it to work for records, but not for unions, yet.
• Use context.ContextualType.Type instead of context.Type, since it is deprecated.