Added documentation for the python classes Point and PersistenceDiagram. Removed the __sub__ method for PersistenceDiagram. Minor renaming. Renamed BottleneckDistance to bottleneck_distance. Renamed intervals iterator for PersistenceDiagram to points. Seems in better conformance with usage.

#include "birthid.h"
#include "optional.h"

namespace dp = dionysus::python;

void export_birthid()
{
    python_optional<dp::BirthID>();   
}