Add new roles: 'gitlab_ci' and 'gitlab_ci_runner'

GitLab CI is a Continuous Integration service that integrats with GitLab
project. More information: https://www.gitlab.com/gitlab-ci/

GitLab CI is divided into two roles:
- 'gitlab_ci' is the main coordination server that is used to configure
  builds;
- 'gitlab_ci_runner' is a simple Ruby script that executes configured
  builds and can be installed on multiple hosts with different settings
  (like access to databases, additional packages to install if required by
  the build process, access to root account via sudo);

GitLab CI Runner cannot currently be automatically registered on GitLab
CI server and requires manual intervention - a needed token can be
copied from GitLab CI "Runners" page and set as a specific variable in
Ansible inventory, which will enable automatic registration process.
Update CHANGELOG.md
Merge pull request #69 from drybjed/role/gitlab_ci

Add support for GitLab CI, update CHANGELOG.md