provide schema data for PcapLoader & PacketNgramRecordReader

implement LoadMetadata interface to provide schema metadata, which is more convenient for pig interactive console. 

```
grunt> A = load 'web.pcap' using com.packetloop.packetpig.loaders.pcap.packet.PacketNgramLoader('tcp:80', '1');
grunt> describe A;
A: {key: long,filter: chararray,bytes_value: chararray,count: int}
```