Provided ability to maintain persistence with IExtendedEntityProperties.

While mod authors are already able to maintain persistence using getEntityData() and the PERSISTED_NBT_TAG, there are some instances in which it seems a persistent extended properties file would be better. I realize that, if considered at all, this request would be better suited for a Minecraft Version update due to the addition of the isDataPersistent() method to the IExtendedEntityProperties interface and the many issues this could arise with current modifications which utilize this interface.