Add pcap-ng format support (Reader only and not full support)

Add pcap-ng file format.
* Reader only and not full support
* File only (not socket stream)
* So, many limitation...please see comment

Refer:
* http://www.winpcap.org/ntar/draft/PCAP-DumpFileFormat.html
* http://wiki.wireshark.org/Development/PcapNg

Will work as below:

```
>>> from pypacker import pcapng
>>> pcr = pcapng.Reader(open("tests/packets_ether2.pcapng", "rb"))
>>> for count, (ts, epb) in enumerate(pcr):
...     print("Packet #{}".format(count))
...     print("  Time:", ts)
...     print("  Interface ID:", epb.interface_id)
...     print("  Capture length:", epb.cap_len)
...     print("  Frame length:", epb.len)
...
Packet #0
  Time: 1417779509.722652
  Interface ID: 0
  Capture length: 90
  Frame length: 90
Packet #1
  Time: 1417779509.722675
  Interface ID: 1
  Capture length: 90
  Frame length: 90
```