Implements attribute label restrictions

[mmagr]

This implements content restrictions on attribute labels, in an effort to make attribute referencing on flows easier for a user.

While doing so, this also updates marshmallow to its latest version, and normalizes outputs (mainly of error results) to be sent as content-type: application/json.

This is connected to dojot/dojot#417

[raulnegreiros]

Why restricts only attribute labels, not all labels?

The commit's content seems good for me.

[mmagr]

Right now, the policy (at least mine), was to limit configuration "as needed".
Since attribute labels are the ones that are actively used across dojot (in flows, and data publishing), it made sense to restrict those.

Device and template labels are currently not used anywhere but on data display (GUI), and thus need not be fettered with system-level restrictions. I for one think that in the future, we should replicate the ID vs label behavior that's implemented for devices and templates, but that would probably be best done on its own PR, since this will probably end up being merged to 0.2.x branch as well in the near future.