[Review] Request from 'aduffeck' @ 'SUSE/machinery/review_140915_migration_infrastructure'

Please review the following changes:
  * b67c970 Add man page for "machinery upgrade-schema"
  * 6e9842e Implement "machinery upgrade-schema" command
  * d9b5dcd Require migrations to describe their purpose
  * a21e267 Lazily load migrations from schema/migrations when needed
  * bc05a25 Implement the Migration base class
  * 7c44955 Extract the JSON loading into a separate method
  * da99739 Create a SystemDescriptionFactory module for the RSpec tests