JSON schema generator: Support enums

[adrianholovaty]

The automatically generated JSON Schema would be better if it included the specific enum values for any type that's an enum. See this comment for some context and an example:

#319 (comment)

[adrianholovaty]

This is now done. I broke it up into a few commits:

• 07ff9a7 — the docs system now has the concept of allowed values for a specific object type.
• 20fd5b0 — the system that auto-generates our JSON Schema now includes enums when available.
• dfd0aa3 — populated the actual enum data for various objects in the spec.