Add Header() method to Packet

Simplifies the API. Unmarshal can now return one Packet value instead
of both a packet and a header.

Relates to #119

pkg/rtcp/packet.go

@@ -2,6 +2,8 @@ package rtcp
 
 // Packet represents an RTCP packet, a protocol used for out-of-band statistics and control information for an RTP session
 type Packet interface {
+	Header() Header
+
 	Marshal() ([]byte, error)
 	Unmarshal(rawPacket []byte) error
 }