Expose basic metadata about the datasets

[pietercolpaert]

Much needed metadata that now is not exposed/configurable:

• When was the dataset last updated? (dct:modified)
• What is the license of the dataset?

Making this obligatory would help understanding some datasets such as whether a TPF interface is using real-time data or whether it uses a stale data dump.

[rubensworks]

Related to #44.

I would not make this obligatory though. I agree it's a good practise, but IMO this server should enable quickly setting up something, without having to pick a license for example.

[pietercolpaert]

Would try to make it default behavior, but agree not going online because you didn't set a license is a bit too harsh :)