Minecraft wiki entry for Player_Heads, associated text:  SkullOwner: The username of the player this is a skull of. This gets converted to the compound version on almost any action.
 SkullOwner: Different definition for the skull's owner.
 Id: UUID of owner, stored as four ints. Optional. Used to update the other tags when the chunk loads or the holder logs in, in case the owner's name has changed.
 Name: Username of owner. If missing or empty, the head appears as a Steve head. Otherwise, used to store or retrieve the downloaded skin in the cache. Need not be a valid player name, but must not be all spaces.
 Properties
 textures
: An individual texture.
 Value: A Base64-encoded JSON object.
 isPublic: Optional.
 profileId: Optional: The hexadecimal text form of the player's UUID, without hyphens.
 profileName: Optional: Player name.
 textures
 CAPE: Optional.
 url: URL of a player cape (64x32 PNG).
 SKIN
 url: URL of a player skin on textures.minecraft.net.
 timestamp: Optional: Unix time in milliseconds.
 Signature: Optional.