Publish and document RequestEncoder interface type

It's internal, but it's a necessarily public type for anyone sending to a
broker. All public types are now consistent (ie they don't take parameters that
are private types, or any such things).

broker.go

@@ -70,7 +70,7 @@ func (b *Broker) Close() error {
 	return b.conn.Close()
 }
 
-func (b *Broker) Send(clientID *string, req requestEncoder) (decoder, error) {
+func (b *Broker) Send(clientID *string, req RequestEncoder) (interface{}, error) {
 	fullRequest := request{b.correlation_id, clientID, req}
 	packet, err := encode(&fullRequest)
 	if err != nil {

request.go

@@ -1,6 +1,8 @@
 package kafka
 
-type requestEncoder interface {
+// An internal interface satisfied by all of the Request structures
+// (MetadataRequest, ProduceRequest, etc).
+type RequestEncoder interface {
 	encoder
 	key() int16
 	version() int16
@@ -10,7 +12,7 @@ type requestEncoder interface {
 type request struct {
 	correlation_id int32
 	id             *string
-	body           requestEncoder
+	body           RequestEncoder
 }
 
 func (r *request) encode(pe packetEncoder) {