* Add StatusNoContent as successful status.

See documentation for expected return statuses: https://docs.gitlab.com/ce/api/README.html#status-codes

Also change the codes to http library codes.

* Change Build Triggers to PipelinTriggers.

This is in line with the updated v4 API. See https://docs.gitlab.com/ce/api/pipeline_triggers.html and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9713 for more information.

* Move Pipeline Triggers to a separate file.

Creates the new service PipelineTriggersService for handling requests to the Pipeline Triggers part of the GitLab API.