Add metadata attribute

[felix-andreas]

Fixes #57

Example:

{
    "version": "2.0",
    "title": "FODO Lattice",
    "metadata": {
        "creator": {"name": "Foo Bar"},
        "editors": [{"name": "Foo2 Bar2"}, {"name": "Foo2 Bar2", "email": "foo@bar.com"}]
    },
    "info": "This is the simplest possible strong focusing lattice.",
    "root": "RING",
    "elements": {
        "D1": ["Drift", {"length": 0.55}],
        "Q1": ["Quadrupole", {"length": 0.2, "k1": 1.2}],
        "Q2": ["Quadrupole", {"length": 0.4, "k1": -1.2}],
        "B1": ["Dipole", {"length": 1.5, "angle": 0.392701, "e1": 0.1963505, "e2": 0.1963505}]
    },
    "lattices": {
        "CELL": ["Q1", "D1", "B1", "D1", "Q2", "D1", "B1", "D1", "Q1"],
        "RING": ["CELL", "CELL", "CELL", "CELL", "CELL", "CELL", "CELL", "CELL"]
    }
}