manage build metadata

@alainjobart 

Burn build-time metadata into our binaries and use this information as a replacement to the startup-time binary-MD5 that we were using for tracking versions before.
I would rework this a bit:
if buildTime != "" {
    t, err := time.Parse(time.UnixDate, buildTime)
    if err != nil panic
    stats.NewInt(...)
}
but that's just a nitpick.