Ensure that the artifacts are stored relative to the top level projects directory


git-svn-id: https://svn.apache.org/repos/asf/buildr/trunk@1527986 13f79535-47bb-0310-9956-ffa450edef68
Add some basic tests around add_exploded_war_artifact


git-svn-id: https://svn.apache.org/repos/asf/buildr/trunk@1527987 13f79535-47bb-0310-9956-ffa450edef68