Track a user who created a pipeline

CHANGELOG

app/models/ci/pipeline.rb

app/models/project.rb

app/models/user.rb

app/services/ci/create_pipeline_service.rb

app/services/create_commit_builds_service.rb

db/migrate/20160715132507_add_user_id_to_pipeline.rb

db/schema.rb

lib/api/commit_statuses.rb

spec/models/ci/pipeline_spec.rb

spec/models/user_spec.rb

spec/services/create_commit_builds_service_spec.rb