* Fix IA_PD length error

I tested this and noticed wireshark wasn't parsing the message correctly. Turns out the length is wrong.

* Update regression.uts

Fix test for DHCP6OptIA_PD

* Update length of DHCP6OptIA_PD in test.

This PR makes it possible to check if several flags are all set, by
writting, for example, `if pkt[TCP].flags.SA:`.

It only works for single letter flags (e.g., will work for TCP().flags
but not for IP().flags), and only when getting a value.

Thanks @guedou for suggesting that.

It is now possible to write `pkt[TCP].flags.S = True`.

This makes it possible to write `if pkt[TCP].flags.S: [...]`.

A new test that triggers the bug (with ARPSourceMACField) has been added.

Fix IPv6 support on Windows

  - LLTDAttribute() produces a Raw instance (or crashes, when
    conf.debug_dissector is set).

  - str(LLTDAttribute(type=18)) crashes.

Thanks @gpotter2

Pypy won't run crypto tests on Travis CI because the cryptography
module cannot be installed with the Pypy version used.

Change the cryptographic library used in the following layers:
 * IPSec
 * TLS
 * Dot11

Notes:
 * Fix the WEP key in regression.uts: Should be a 40 bits key (5 ASCII)
   and 24 bits nonce for 64 bits WEP. The key was 8 ASCII => 88 bits
   in total
 * AES-GCM and AES-CMAC algorithms are now native in cryptography: add
   tests in ipsec.uts
 * cryptography only supports Python 2.6+
 * Fix the docstring for the pad method

Removed:
 * MD2 and MD4 hash algorithms for RSA in tls/crypto/pkcs1.py
 * AES-XCBC-96 not supported by cryptography.

Signed-off-by: Thomas Faivre <thomas.faivre@6wind.com>

One can now use tcpdump or tshark to dissect packets from
Scapy. This can help, for example, to check Scapy's dissectors

This fixes at least an issue fragmentation (for which a new test
has been added)

* Add conf.checkIPinIP

This setting, when set to False, will exclude IP layers which encapsulate
another IP layer from the checks to tell if a packet is an answer to
another one.

Fixes #383

* conf.checkIPinIP affects both IP() and IPv6()

Setting conf.checkIPinIP will handle IP()/IP(), IP()/IPv6(),
IPv6()/IP(), IPv6()/IPv6().

Thanks @guedou!

* Clean-up IP.hashret and .answers code

* Add tests for .hashret() and .answers()