Add VXLAN backend

VXLAN datapath is supported by the kernel thereby reducing the
overhead of TUN device and userspace switching. This patch takes
advantange of DOVE extensions and does not use multicast for port
flooding.

Fixes #18
Could that then relate to the other point about flannel.x -- ie, that there should be one (and only one) flannel running at a time? It may not solve the update issue, but detecting other flannels on a single box (lock file, or somesuch) and bailing might help. As for updates (without losing connectivity, temporarily) that seems harder. Don't know, just thinking out loud.