Mention supported operating system in README
Make specific os detection methods private, introduce "#get_os" instead

"OsInspector#get_os" is the public method that should be called in order
to retrieve the Os object. It cascades the different detection methods
for specific operating systems which makes it easier to use and extend.
Add helper method which ensures that we're running on a compatible host
Abort Machinery when we're running on an incompatible host
System#cat_file -> #read_file, make it os agnostic on local systems

System#read_file used "cat" internally which means that it didn't work on
MacOS and Windows and was also harder to test.
Refactor OsInspector

It now uses System methods instead of implementing its own stuff. That
makes it cleaner and easier to test.
Make OsInspector#get_os private, #inspect is the API to be used
Rename Os#can_run_machinery to Os#can_run_machinery?
Move checks for host capabilities to LocalSystem and name them properly