[Review] Request from 'tgoettlicher' @ 'SUSE/machinery/review_141217_exclude_mkinitrd'

Please review the following changes:
  * f44b59a Exclude mkinitrd files from export
  * 1ba3c29 Fix long lines
  * 06ff6db Extract JSON handling into a Manifest class
  * 2d52611 Get rid of SystemDescription#from_json method
  * d8a0df4 Split #from_json into three methods (parse, validate, objectify)