JsonSchemaFactory: add comments

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Add JsonSchema class

This class has the same API, or nearly so, as the old JsonSchema. The
differences are it can now return a boolean instead of a report for indicating
success/failure.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>