Network metadata/metrics compatible with older server versions Fixes #128

This probably needs some refactoring, but it sets the iface for windows network metrics and is compatible with older versions of windows server

This is currently running on ny-web01 pointing to dev bosun. See os.net.bytes at http://goo.gl/80uLzZ

See the redis collector for how it manages a dynamically changing list of servers to monitor: https://github.com/bosun-monitor/scollector/blob/c2a840cdebb6050366d210f73af3f17652773e39/collectors/redis_unix.go#L96

We should do the same thing here.