HigherPower - Improving /genhere

After noticing the setRandomHeight method of the StructureStart class, I decided to look into it.  It appears to be the way normal Minecraft provides variation in the height of Nether fortresses, strongholds, and abandoned mineshafts.  I used the location of the command sender to make the structure generate at a similar Y level to the player.  It has the unfortunate side effect of making villages fail to generate, which I will fix somehow sometime.