Support zerocopy + bugfix

* NewPacket2 supported DLT_EN10MB packets, and DLT_LINUX_SLL seemed to work most of the time... 95% of the time in a test system, with 5% packet loss in live tests.
* LoopWithCallback as an alternative to NextEx2.  It enables the zerocopy path of libpcap 1.5.0+, even if it's a little more cumbersome to use.

ping @ElPeque 
Did some tests... long post
```
package main

import "time"
import "log"

type TcpPacket struct {
	DstAddr   uint32
	SrcAddr   uint32
	AckSeq    uint32
	Seq       uint32
	Source    uint16
	Dest      uint16
	Flags     uint16
	Payload   []byte
	Timestamp time.Time
	IsRequest bool
	saved     bool
}

func retPktPTR() *TcpPacket {
	var packet TcpPacket
	packet.Seq = 1
	return &packet
}

func retPktVAL() TcpPacket {
	var packet TcpPacket
	packet.Seq = 1
	return packet
}

const iter = 500000000

func main() {
	var i, t, cnt uint32
	var start, end, accP, accV int64

	retPktPTR()
	retPktVAL()

	for t = 0; t < 3; t++ {
		start = time.Now().UnixNano()
		for i = 0; i < iter; i++ {
			cnt += retPktPTR().Seq
		}
		end = time.Now().UnixNano()
		accP += end - start
		log.Println("PTR time", end-start)

		start = time.Now().UnixNano()
		for i = 0; i < iter; i++ {
			cnt += retPktVAL().Seq
		}
		end = time.Now().UnixNano()
		accV += end - start
		log.Println("VAL time", end-start)
	}

	log.Println("cnt", cnt, "VAL/PTR", float32(accV)/float32(accP))
}
```
$ go build ptrbench.go ; sync ; sleep 3 ; sudo nice -10 ./ptrbench
2014/09/17 11:12:07 PTR time 3300181961
2014/09/17 11:12:18 VAL time 10706706612
2014/09/17 11:12:21 PTR time 3382147536
2014/09/17 11:12:32 VAL time 10778394664
2014/09/17 11:12:35 PTR time 3255079470
2014/09/17 11:12:46 VAL time 10719739447
2014/09/17 11:12:46 cnt 3000000000 VAL/PTR 3.2407684

After removing time.Time: VAL/PTR 3.2730277
Also removed []byte: VAL/PTR 4.9653754
Only Seq left: VAL/PTR 1.2771899

Bottom line: *struct is always faster than copy.

Additional test:  type TcpPacket uint32 ... plus other need changes: VAL/PTR 0.9953344