Sample dataset before generating batch order
Sample dataset before generating batch order

Fixes #227.

monsieurzhang/OpenNMT,jsenellart-systran/OpenNMT,monsieurzhang/OpenNMT,jsenellart/OpenNMT,da03/OpenNMT,monsieurzhang/OpenNMT,jungikim/OpenNMT,jungikim/OpenNMT,OpenNMT/OpenNMT,jsenellart/OpenNMT,da03/OpenNMT,da03/OpenNMT,jungikim/OpenNMT,jsenellart-systran/OpenNMT,jsenellart-systran/OpenNMT,OpenNMT/OpenNMT,OpenNMT/OpenNMT,jsenellart/OpenNMT