Rename "build_service" directory to "obs"

The new name is more consistent with the constant name used in code and
terminology used in README.md ("OBS directory"). It is also shorter.
Remove "yast" subdirectory from killer's data directory structure

The "yast" directory was the only subdirectory in killer's data
directory and as such it was pretty much useless. Since the data
directory structure is now relatively stable, this state probably won't
cahnge. Let's get rid of the directory.
Merge pull request #386 from mvidner/master

yk test: use rb instead of ybc for dependencies.
Merge pull request #384 from yast/directory-changes

[Review] Request from '' @ 'yast/ycp-killer/directory-changes'
packaging - remove all files from OBS checkout before copying

to get rid of obsoleted files
Merge pull request #387 from lslezak/package_cleanup

packaging - remove all files from OBS checkout before copying
README.md: Make "Module Metadata" a level 2 heading

It doesn't really belong to the "Usage" section. This change also puts
the "Module Metadata" section to the same level as "New YaST Module
Structure", which is kind of similar (both are essentially references).
README.md: Reorder command descriptions alphabetically
README.md: Add stubs for "yk build" and "yk submit"

Content will be filled-in later.
README.md: Add "Contents" section
Merge pull request #388 from yast/readme-structural-improvements

[Review] Request from '' @ 'yast/ycp-killer/readme-structural-improvements'
README.md: Update "yk help" output to match reality
README.md: Improve the "Usage" section

  * Remove redundant sections from the intro text, move everything into
    one place.

  * Change terminology: s/subcommand/command/

  * Use four spaces instead of ``` to delimit "yk help" output. It is
    more readable in plaintext.

  * Other small improvements.
README.md: Remove examples of executed commands

I don't think they are very helpful as they all look more or less the
same. They also create maintenance burden as they need to be updated
when command output changes.
Merge pull request #389 from yast/readme-usage-improvements

[Review] Request from '' @ 'yast/ycp-killer/readme-usage-improvements'
README.md: Fix indentation
README,md: Rewrite command descriptions

The goals of this change are:

  * to make the command descriptions as clear as possible
  * to make them agree terminologically
  * to make them agree stylistically
  * to bring them on the same level of detail
  * to sync them with code

All the descriptions were written from scratch. After that, they were
cross-checked with the code and the old descriptions to make sure
nothing important was missed.
Update patch for the "mail" module to match upstream
Merge pull request #391 from yast/update-mail-patch

Update patch for the "mail" module to match upstream
Stub MasterCFParser.pm

Stubbed because otherwise the "mail" YaST module will attempt to load
this file from the system, where it is broken.