support Zipkin v2 Span format

[flier]

Zipkin v2 add a new span format (openzipkin/zipkin#1499)

[flier]

hi @adriancole,

could you help to review the changes? the test output like:

{
    "traceId": "1cc0b75520b978820a01f5444929a73a",
    "name": "test",
    "id": "fe23dadb5d05a7d9",
    "parentId": "b2f8d060661abe18",
    "kind": "CLIENT",
    "timestamp": 1502430154383331,
    "annotations": [
        {
            "timestamp": 1502430154383335,
            "value": "cs"
        }
    ],
    "tags": {
        "sa": "8.8.8.8",
        "bool": "1",
        "i16": "123",
        "i32": "123",
        "i64": "123",
        "double": "12.3",
        "string": "测试",
        "bytes": "AQID"
    }
}

[nicksbyman]

Hi @adriancole and @flier do you both have a plan for merging #12? Do either of you want some help?