support google timestamps, use date-time format in json schema

[huttotw]

The well known type google/protobuf/timestamp.proto will now be represented as:

{
    "$schema": "http://json-schema.org/draft-04/schema#",
    "properties": {
        "timestamp": {
            "type": "string",
            "format": "date-time"
        }
    },
    "additionalProperties": true,
    "type": "object"
}

[chrusty]

@huttotw I'm sorry this slipped through the cracks. I guess one of the refactoring PRs made this too difficult to resurrect, so I raised a new PR from your work: #25

I'll credit you in the release notes, sorry to have missed it. I think I disabled GitHub notifications because I belong to some busy GitHub orgs from time to time which spam my inbox.