Originate: change key to referenceKey (#99)

Having the "key" field to Originate and StageOriginate is confusing.  It
is easy to think that you should pass in the key for the channel you
wish to create, rather than it being a completely optional "originator"
key.  Therefore, this patch changes the name of the variable to
`referenceKey` so as to disambiquate the two, as well as offering an
explanation in the comments.

client/native/channel.go

@@ -35,7 +35,7 @@ func (c *Channel) List(filter *ari.Key) (cx []*ari.Key, err error) {
 	return
 }
 
-// Hangup hangs up the given channel using the (optional) reason
+// Hangup hangs up the given channel using the (optional) reason.
 func (c *Channel) Hangup(key *ari.Key, reason string) error {
 	if key == nil || key.ID == "" {
 		return errors.New("channel key not supplied")
@@ -71,9 +71,13 @@ func (c *Channel) Get(key *ari.Key) *ari.ChannelHandle {
 	return ari.NewChannelHandle(c.client.stamp(key), c, nil)
 }
 
-// Originate originates a channel and returns the handle
-func (c *Channel) Originate(key *ari.Key, req ari.OriginateRequest) (*ari.ChannelHandle, error) {
-	h, err := c.StageOriginate(key, req)
+// Originate originates a channel and returns the handle.
+//
+// **Note** that referenceKey is completely optional.  It is used for placing
+// the new channel onto the correct Asterisk node and for assigning default
+// values for communications parameters such as codecs.
+func (c *Channel) Originate(referenceKey *ari.Key, req ari.OriginateRequest) (*ari.ChannelHandle, error) {
+	h, err := c.StageOriginate(referenceKey, req)
 	if err != nil {
 		return nil, err
 	}
@@ -82,9 +86,13 @@ func (c *Channel) Originate(key *ari.Key, req ari.OriginateRequest) (*ari.Channe
 
 // StageOriginate creates a new channel handle with a channel originate request
 // staged.
-func (c *Channel) StageOriginate(key *ari.Key, req ari.OriginateRequest) (*ari.ChannelHandle, error) {
-	if key != nil && req.Originator == "" && key.Kind == ari.ChannelKey {
-		req.Originator = key.ID
+//
+// **Note** that referenceKey is completely optional.  It is used for placing
+// the new channel onto the correct Asterisk node and for assigning default
+// values for communications parameters such as codecs.
+func (c *Channel) StageOriginate(referenceKey *ari.Key, req ari.OriginateRequest) (*ari.ChannelHandle, error) {
+	if referenceKey != nil && req.Originator == "" && referenceKey.Kind == ari.ChannelKey {
+		req.Originator = referenceKey.ID
 	}
 
 	if req.ChannelID == "" {