Update to header structure

+-------------------------------------------+
| prefix | header | gap | payload | postfix |
+-------------------------------------------+
JHeader.StateJHeaderJHeader
hasPrefix():boolean, getPrefixOffset():int,
getPrefixLength():int, getPrefix():byte[]
+------------------------------------------------------------------------------+
| preamble | ethernet | ip4 | udp | rtp | payload | rtp_padding | ethernet_crc |
+------------------------------------------------------------------------------+
0          8          22    42    50    62        162           163            167
+------------------------------------------------------------------------------+
|          | ethernet | ip4 | udp | rtp | payload | unused packet part         |
+------------------------------------------------------------------------------+
0          8          22    42    50    62        162           163            167
       +------------------------------------------------------------------------------+
Eth => | preamble | ethernet | ethernet_payload                        | ethernet_crc |
       +------------------------------------------------------------------------------+
Ip4 => |                     | ip4 | payload                           |              |
       +------------------------------------------------------------------------------+
Udp => |                           | udp | payload                     |              |
       +------------------------------------------------------------------------------+
Rtp => |                                 | rtp | payload | rtp_padding |              | 
       +------------------------------------------------------------------------------+
       0          8          22    42    50    62        162           163            167
JHeader.getPrefix():byte[]typeHeaderLength.Type@Header(prefix = 8)@HeaderLength(HeaderLength.Type.POSTFIX)

Changes checked in. Also