Server Streaming RPC support

[Xorlev]

Server streaming is implemented as a stream of newline-delimited
protojson entries. Messages are flushed to a ChunkedOutput (Jersey only,
StreamingOutput was giving me trouble) upon receipt from the server
stub. As HTTP/1.1 lacks wide support for trailers, the only realistic
way to signal mid-stream error is to emit an error struct (or hang up). On error, a
google.rpc.Status proto is emitted and the request is ended, e.x.

< HTTP/1.1 200 OK
< Date: Sat, 11 Mar 2017 20:59:01 GMT
< Vary: Accept
< Content-Type: application/json;charset=utf-8
< Vary: Accept-Encoding
< Transfer-Encoding: chunked
<
{"request":{"s":"23","int3":40}}
{"code":15,"message":"HTTP 500 (gRPC: DATA_LOSS): Terribly sorry, a grue ate your data. :("}

Additionally, stream handlers support responding to the text/event-stream
MIMEType. When encountered, grpc-jersey emits data: {protojson}\n\n
for compatibility with general ServerSentEvents, allowing use by
EventStream in browser JavaScript.

All error handling is done through Jersey{Unary,Streaming}Observer now, which delegates error handling to an instance of GrpcJerseyErrorHandler. This instance can be statically set at runtime (ideally before requests begin) by calling ErrorHandler#setErrorHandler, e.g.:

GrpcJerseyErrorHandler errorHandler = new MyErrorHandler();
ErrorHandler.setErrorHandler(errorHandler);
// Go on with setup...

Punted to another PR:

• Client streaming
• Bidi streaming (or, at least what can be done without HTTP/2 or Websockets)