Packet().overload_fields can be overwritten

As suggested by @guedou, this adds tests for fragment() and
defragment() functions.

This is needed by fragment(), which has been broken since PR #11.

Added AEAD digest support to IPSec layer, specifically for GCM and CC…

Add contributing guidelines

TracerouteResult.world_trace() now uses Matplotlib Basemap Toolkit

Fix TracerouteResult.world_trace() and set new defaults for matplotlib graphs

Support LLTD protocol

Fix conf.debug_dissector in regression tests

Use conf.iface MAC address by default when Scapy cannot find the route

Fix Num2Layer.__repr__() (broken after PR #11)

Windows: use Winpcap, drop DNET dependency

self-merge since @guedou is OK and @n1nj4sec agrees.

Use the provided interface in attach_filter()

This is related to the e-mail thread[1] on Scapy mailing list, based
on the proposed PR on Bitbucker Suraj Deshmukh [2].

[1] http://article.gmane.org/gmane.comp.security.scapy.general/5297
[2] https://bitbucket.org/secdev/scapy/pull-requests/127/issue-5121-now-iface-option-of-sniff/diff