* layers/vxlan: add group policy extension support
This is based on the following draft RFC:
http://tools.ietf.org/html/draft-smith-vxlan-group-policy-00



Add more unit tests in regression.uts.

Signed-off-by: Robin Jarry <robin.jarry@6wind.com>

* Move VXLAN to layers

VXLAN has an official RFC: http://tools.ietf.org/html/rfc7348


It should be in automatically loaded layers.

Move tests to regression.uts.

Signed-off-by: Robin Jarry <robin.jarry@6wind.com>

* load_contrib: fallback to layers when contrib not found

Useful when a scapy.contrib module has been moved to scapy.layers.

Signed-off-by: Robin Jarry <robin.jarry@6wind.com>