Importing and Exporting AST to and from JSON

[pmbittner]

The ASTs from issue #50 have to be serializable sucht that we can compare them across sessions and between tools. Therefore, we agreed on a common JSON format:

{
    "uuid" : 1,
    "value" : "some string",
    "grammar_type" : "grammar type as string",
    "children" : [
        "... further nodes in recursive structure"
    ]
}

So the goal is, to implement and exporter and an importer to serialize ASTs into that format.