Add entity drawing to drawing decorator

[DaveyBiggers]

At the moment, we can use the drawing decorator to spawn items and place blocks, but there is no way to place entities. Adding a DrawEntity command would open up a large number of possibilities for experiments.

At its most basic, we can simply specify the x/y/z and entity type - but we should possibly allow for greater control, maybe even to the level allowed by the Minecraft summon command, which can take JSON - eg:

/summon Guardian ~0 ~1 ~0 {Motion:[0.0,1.0,0.0],Attributes:[{Name:generic.movementSpeed,Base:1.0}]}

(see, for example, http://www.minecraftupdates.com/summon-command )