Merge tag '2.2' into develop

Version 2.2
Merge branch 'master' into develop
Merge branch 'master' into develop
Merge branch 'master' into develop
Handle lost empty directories while creating symlinks.
Prefer dist when changing the required contao/core version.
Refactoring: extract if ($this->io->isVerbose()) {$this->io->write(...); }.

We now have a write() and a writeVerbose() method in AbstractInstaller.
Activate ccabs.
Change tabs to spaces.
Re-add composer/composer to require-dev.
Add missing phpDoc blocks.
Fix code style issues.
Set php version in composer.json and add 5.6 to build environments.
Bump dependencies.
Hotfix #10, inconsistence directory separator on windows.

Also fixes contao-community-alliance/composer-client#227
Merge branch 'release/2.2.3'