NetworkService: changed API to support more backends (namely wicked)

The change is backward incompatible
Updated changelog
Document and test AsciiFile

I want to use it in yast2-nfs-client to preserve comments (bnc#429326)
as it is already used elsewhere for fstab handling.
deprecate yast -i (FATE#316458)
Changes
adapt to changes suggested by mvidner
improve changelog entry
Merge pull request #158 from yast/deprecate_yast_i

Deprecate yast i
First draft of hook main doc
Add more about debugging
Update content
More updates
Formatting
Add readme into the var lib hooks dir
Review fixes
Add output method to every hook file which merges stdout and stderr
Remove list of hooks as they are too specific to be included here
Add STDERR and STDOUT description to file hook output
Hoke file output rather returns empty string than nil
Update changes, spec, version 3.1.9