Implement meta-metadata structure for JSON dumps

It became evident that putting things like identifier and date only into
the filename is a bad idea for storing meta-metadata. Although this was
clear from the beginning I am fixing this now. I fix it by simply using
an additional layer.

All the metadata are now stored inside another JSON object. This looks
like the following:

{
    "id": "GovData.de",
    "date": "2013-09-16",
    "metadata": [...]
}

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>