Rakefiles: refactor Jenkins API for downloading (binary) artifacts ...to allow multiple "backends" in (near) future. Introduce a concept of an arfifact repository. A repository contains multiple builds (currently the API provides only access to `latestBuild()`), build contains multipe artifacts. An artifact is essentially a downloadabe file. This commit is a preparation for introduction of "simple HTTP listing"-based repository as there's no public Jenkins instance anymore.

syntax: glob

build
artifacts
reports
tmp
config.rb
.rake_tasks~