Use case 1: Initial validation

[jakubklimek]

We will start with our minimal use case and add complexity as we go.
Let us use this descriptor.
The validator should:

1. Access the descriptor using its URL
2. Warn when the HTTP content-type is different from application/csvm+json; charset=utf-8
3. Determine that the tableSchema is another resource and download it (warn again)
4. Access the CSV file using url - this should be streamed, as it can be very large
5. Warn when the HTTP content-type is different from text/csv; charset=utf-8
6. Read the CSV file header (first row) - this will be optional later
7. Parse tableSchema.columns array and verify presence of required columns indicated by titles - this can be also an array in future, but start with a simple literal.