Create Object model system, used for changing models.

Mainly used for spells that change the player's model, such as Nidalee's R. I included a simple version of her ult (although it should suffice, as that is really all her ult does). Turrets currently have a `""` model, because I was not sure on how to implement that. For minions, I was assuming `Red_Minion_MechCannon` was the cannon minion, because I could not find any others in [here](http://i.imgur.com/UujcorB.png). The model system might want to be moved one layer up to `Unit` instead of `Object`, but this seems to work for now.