Added a simple json writer.

Class Json
Gives you a json encoded file. Every id of a vertex in the graph is a root identifier in the json file. Every first
level vertex (it has an edge TO the vertex) is in the collection to that identifier. E.g.:

{
   "firstThing" :
      [
          "dependencyOfFirstThing1",
          "dependencyOfFirstThing2"
      ]
}