API: Add API endpoint to return message headers

See #15

server/websockets/hub.go

@@ -7,7 +7,6 @@ package websockets
 import (
 	"encoding/json"
 
-	"github.com/axllent/mailpit/data"
 	"github.com/axllent/mailpit/logger"
 )
 
@@ -27,6 +26,12 @@ type Hub struct {
 	unregister chan *Client
 }
 
+// WebsocketNotification struct for responses
+type WebsocketNotification struct {
+	Type string
+	Data interface{}
+}
+
 // NewHub returns a new hub configuration
 func NewHub() *Hub {
 	return &Hub{
@@ -68,7 +73,7 @@ func Broadcast(t string, msg interface{}) {
 		return
 	}
 
-	w := data.WebsocketNotification{}
+	w := WebsocketNotification{}
 	w.Type = t
 	w.Data = msg
 	b, err := json.Marshal(w)