specify option to limit notifications

[ci skip]
more concise parameter names

[ci skip]
Add separate npm Karma script, invoked by npm test

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge pull request #2102 from opf/feature/specify_optional_mail_notification

[API] Specify optional mail notification
Add ruby 2.3.1 to setup
Update ruby version to current stable 2.1.4

This release includes security fixes for the following vulnerabilities:

* CVE-2014-8080: Denial of Service XML Expansion
* Changed default settings of ext/openssl related to CVE-2014-3566

And there are some bug-fixes.
Add debian/ubuntu installation guide

This way we can bind our documentation to the source code.
Things will never get out of sync and we can (hopefully) sometime
automatically take this documentation and put it on a fancy help page
somewhere.
Merge pull request #2125 from opf/hotfix/set_ruby_version_for_pachager

Hotfix to set ruby version for packager
Merge branch 'release/4.0' into dev
use private sprockets fork

The sprockets maintainer yanked the sprockets version we
use from rubygems. It happened that I had the specific sprockets version
at my harddrive.

We really should update sprockets, but until this happens, the fork can
be used.
Merge pull request #2128 from opf/fix/sprockets_backports

Use Private Sprockets Fork
fix not apparent which dropdown items can be activated
use private sprockets fork

The sprockets maintainer yanked the sprockets version we
use from rubygems. It happened that I had the specific sprockets version
at my harddrive.

We really should update sprockets, but until this happens, the fork can
be used.
Merge pull request #2124 from opf/hotfix/user_guide

Add Debian/Ubuntu Installation Guide
Merge branch 'release/4.0' into dev
Merge pull request #2130 from opf/hotfix/yanked_sprockets

use private sprockets fork
Merge pull request #2113 from opf/fix/add-npm-run-karma

Add separate npm Karma script, invoked by npm test
Add packager installation guide

Also moved manual installtation guide to operation_guides
to be more flexible which kind of documentation will be
placed into this folder.
Minor changes of style
Merge pull request #2131 from opf/hotfix/packager_installation_guide

Add packager installation guide.