Created a new Graph class.
Created a new Vertex class.
Added import statements to reflect class dependencies.
Added private vertices property in Graph class. Lazily instantiating it.
Made the vertices property in the Graph class public.
Forgot to make the vertices property strong.
Added public adjacentVertices property in Vertex class. Lazily instantiating it.
Implemented public addVertex: and removeVertex: methods in Graph class.
Added public addAdjacentVertex: and removeAdjacentVertex: methods in Vertex class.
Fixed bugs in addAdjacentVertex: and removeAdjacentVertex: methods.
Added some test code in AppDelegate class.