Add timezone configuration to gitlab.yml

This pull request refer to this suggestion: http://feedback.gitlab.com/forums/176466-general/suggestions/5890076-add-timezone-configuration-to-gitlab-yml

It's now possible to set time zone form gitlab.yml configuration file by adding a little time_zone initializer.
Yes, when you start GitLab, the workflow is:
* Set default time-zone (UTC)
* config.time_zone on application.rb ? Override
* time_zone on gitlab.yml ? Override