Add required option to field definitions

[leplatrem]

Allows to specify optional fields in definition.

I opend this PR, just to make sure you're ok with the required keyword at field level.

"fields": [{"name": "age", "type": "int", "required": False}]

If we switch to JSON schema #53, we'll move to global required attributes with list of field names, at definition level.

"fields": [{"name": "firstname", "type": "string"}, {"name": "age", "type": "int"}],
"required": ["name"],

The latter is slightly more complicated in client-side form generation...

[almet]

I prefer the former, but JSON Schema is probably a good idea, since they already define the format, so complying with their APIs is IMO the way to go, nope?

[leplatrem]

Ok then.

We keep the former.

And think again about this once we implement #53.

You can merge if you like then.