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
Took me a second. Would these cases be better handled (if they're mutually exclusive like this) by len(miss.IP) != 0 { handleL3Miss(...) }  -- suggesting the presence of one instead of the absence of the other?