[WIP] Making AddStats() accept multiple parameters.

This PR follows up the discussion on #200. It changes the StorageDriver interface so that ``AddStats()`` will accept multiple stats making the underlying implementation possible to do batch writes.
@vishh What do you mean by splitting the API? Do you mean we need to add ``AddSingleStats(ref info.ContainerReference, stats *info.ContainerStats) error`` into the memory storage? I think this function could be implemented outside the ``StorageDriver`` interface as ``AddSingleStats(driver StorageDriver, ref info.ContainerReference, stats *info.ContainerStats) error``.