Implement an extensible deserialization scheme that can work with tmx objects

[thelukester92]

So that entities can be stored and loaded from files, implement a deserialization scheme that component classes can implement. One possible way is to register component deserialization methods in the scene based on a unique string. For example, this JSON could represent an entity:

{ "position" : { x: 12, y: 24 } }

The LGPosition component could register for it like this in the scene:

registerDeserializer("position") { LGPosition.deserialize($0) }

And the result of deserialization would be this:

LGPosition(x: 12, y: 24)

Perhaps a Deserializable protocol could work, then registering is simply associating a string with a class. It would be nice if Swift let us use Types as Dictionary keys and values.

[thelukester92]

#26 implements #25, which will add the ability for the tmx parser to load tmx <object> elements into memory. These elements have a type attribute that could indicate some kind of template, as well as custom property key-value pairs that could indicate components like this (similar to the above example):

Key	Value
position	{ x: 12, y: 24 }
sprite	{ spriteSheet: "Player" }

As proposed before, LGPosition and LGSprite would register to be deserializers for "position" and "sprite" respectively.

Better yet, with the possibility of using type as a template indicator, the game could define templates in a json object somewhere and just use the property values to override or add to the template-created entity.