[Review] Request from 'dmajda' @ 'SUSE/machinery/global-json-schema'

Please review the following changes:
  * c41e4f2 Implement JSON Schema validation of system descriptions
  * f09366e Don't save version metadata for descriptions without format version
  * bb6ac76 Extract SystemDescription#validate_json
  * c683b0a Use "class << self" syntax for methods in SystemDescription
As far as I could see, this will make the `ensure_compatibility!` check in `system_description_store.rb` redundant as old system descriptions are already detected here. It also fixes #110 albeit with a rather ugly error message.