Add OutPacket interface and ByteBuf implementation

Replacement for LittleEndianWriter, GenericLittleEndianWriter,
and MaplePacketLittleEndianWriter.

src/main/java/net/packet/Packet.java

@@ -1,5 +1,6 @@
 package net.packet;
 
 public interface Packet {
+    short getHeader();
     byte[] getBytes();
 }