Implemented IDs for Critters, some of client

-Implemented IDs for Critters
-Implemented critters/GET and critters/POST
-Implemented critter/GET/id and critter/DELETE/id

-added JsonCritter class to use
-added method to add critter using a JsonCritter in World

-A bit of confusion on the two options for Critter POST
-still working on world/POST, world/GET/update_since

Eventually, my plan is to remove all instances of world in the GUI class
so that every thing is taken care of in the Server