Add Device ID Field to WRP SimpleEvent and RequestResponse

[kristinapathak]

wrp-go/messages.go

Line 315 in 435f063

type SimpleEvent struct {

The Source of a SimpleEvent is no longer always the device ID, so to provide an easy way to understand what device the event is for, we will add a separate field for that. The field should also be added to the general wrp.Message struct and the RequestResponse struct, so that Device ID can be added for responses.

This update will match the new spec after xmidt-org/docs#51 is fixed.

[kristinapathak]

The new field should also be added everywhere applicable, such as the translation of struct to headers:

wrp-go/wrphttp/headers.go

Line 231 in 435f063

func AddMessageHeaders(h http.Header, m *wrp.Message) {