Strict JSON schema for orders

[ralienpp]

This pull request aims to address this issue #10 and it adds a few improvements. It creates a better "safety cushion", by making the JSON schema more strict. This enables us to easily and immediately catch some types of errors.

changes:

• is refactored a bit, to avoid duplication and facilitate reuse
• provides comments in some tricky places to make it easier to follow the schema's logic
• enforces the presence of at least 1 node in an order
• enforces an ISO8601 timestamp format
• enforces the presence of actionParameters in the cases when actionType is a standardized action which has mandatory parameters (actions without parameters and proprietary actions are not going to be verified)

Although this update makes the schema better, one should still perform post-hoc validation when complex logic is involved.