job can now query Jenkins to load ALL its builds

By default, Jenkins API only give the last 100 builds of a job (since this commit : https://github.com/jenkinsci/jenkins/commit/fe9f67 ).

This pull request adds a method Job#load_all_builds. Calling load_all_builds will fetch build number and url of all job builds and store them in job._data['builds'], replacing the previous value and preserving behaviour.