Improve VXLAN layer

Try to read the manufacturer DB from various places

Fix DLT_* constant values

[coverage] Full rework of IGMP(v1/2/3) + tests

Improved PR#751

[Python 2 to 3] Bytes fixes

TUN/TAP Interfaces, improve & test bridge_and_sniff()

Add a fancy prompt

Updated README.md

[coverage] Add ScapyPipes tests

[coverage] Reduce coverage randomness: tls+TEX

Make FlagValue objects hashable