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
It is intended but it's definitely contentious. The name will be flannel.X so it should be ours but one never knows. The alternative is to error out and have the user delete the interface but after thinking about it, I decided that it was an inferior solution. However, thoughts and ideas on how to handle configuration changes/upgrades better are welcome.