[Review] Request from 'aduffeck' @ 'SUSE/machinery/review_141016_refuse_to_run_on_incompatible_hosts'

Please review the following changes:
  * 30742e9 Refactor OsInspector
  * a691adf System#cat_file -> #read_file, make it os agnostic on local systems
  * 8749dc4 Abort Machinery when we're running on an incompatible host
  * 1c97942 Add helper method which ensures that we're running on a compatible host
  * 1ba4efc Make specific os detection methods private, introduce "#get_os" instead
  * 07942d1 Mention supported operating system in README
Avoid using {...} for multi-line blocks.