* Use "EncodeMessage" instead of "Marshal" in serve<methName>Protobuf so message length is written properly
* Flush stream response after each message if response writer implements http.Flusher
* Add streaming "MakeHats" endpoint to example service

* Also, add tests for example server
* JSON clients currently fail to stream (rpc call gets EOF err instead of RespStream)

* Add new "test_example" target to Makefile. Example tests are passing for protobuf clients; JSON tests would fail, but are commented out
* Return normal twirp errors from download requests that fail before the stream is created
* Consolidate generator logic for unary and download rpc types in generateServerProtobufMethod
* In the example server, use chan of HatOrError type to send mid-stream errors

…ity with older versions of go

* Define `<Type>OrError` struct that is a union of the `<Type>` and `error` return values from `Next`
* Define `New<Type>Stream` constructor that takes a channel of `<Type>OrError` and returns an implementation of the `<Type>Stream` interface

…tyle rpcs