Support date types

[i4ki]

For now that we have metadata associated directly with documents we can implement this feature.

The usage can be like below:

    idx := index.New(...)

    metadata := index.Metadata{
        "updateAt": index.Metadata{
            "type": "date",
            "format": "YYYY-mm-dd",
        },
    }

    idx.Add(1, []byte(`{"name": "John Doe", "updateAt": "2015-06-29"}`), metadata)

See: #25