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.
example name; better name: stub_file_reading
improved rspec style much
Merge branch 'master' into asciifile-usage

Conflicts:
	library/general/test/Makefile.am
AsciiFile: improved documentation
fixed the yardoc warnings

caused by incomplete doc comment conversion (implicit param names)
AsciiFile spec: better descriptions

it is more fragmented now, like the result that it specifies :)
Merge pull request #149 from mvidner/asciifile-usage

Document and test AsciiFile
Merge pull request #180 from vmoravec/add-systemd-socket

Add systemd socket support | bnc#853300
Update changes and version to 3.1.20