make generated json for DateTime and UUID private

- currently they can be imported and conflict with existing,
  implicits for the DateTime and UUID types.
prefix generated json to avoid collision on import

- If apis A and B both declare a model named C, then importing

        a.models.json._

  and

        b.models.json._

  would mean importing conflicting implicits named

        writesC

  name implicits according to

        jsonWrites<camel_case_service_name><camel_case_model_name>
  and
        jsonReads<camel_case_service_name><camel_case_model_name>

  to avoid this.
Merge pull request #75 from gilt/mreed-json-collision-fix

Mreed json collision fix