CDK-375: Use logger for console interaction.

This commit uses a slf4j logger for console interaction rather than
using System.err directly. This enables verbosity levels and
config-based control of messages to users.
CDK-375: Add schema commands.

Commands:
* schema: output the schema for a dataset
* csv-schema: create a schema from a csv data sample
Merge branch 'CDK-375-add-schema-commands'