Support additionalProperties other than 'true'

[danopia]

For a key/value map structure, like {"web":8000,"api":8001}, I can't find how to apply a schema to the values regardless of key. Ideally a full object schema could be applied to specify a rich map data structure.

ref: https://stackoverflow.com/questions/22229250/how-to-create-json-schema-for-name-value-structure#comment60712837_22232419