Spindly trees, rusted gates, crumbling stone, a solitary mourner: these things come to mind when we think of cemeteries. But not long ago, many burial grounds were lively places, with gardens and crowds of people -- and for much of human history, we didn't bury our dead at all. How did cemeteries become what they are today? Keith Eggener delves into our ever-evolving rituals for honoring the dead