Add conf.checkIPinIP (#387)

* 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()