Allow either RunExecution (flyweight task) or AbstractBuild (with workspace)

See notes for commit 444097d65ecb97c92005b3acd63db8e03bdeb6a3.
Makes job implementation be configurable. One option allow to use a workspace.
Merge pull request #45 from dnozay/configurable

Allow either RunExecution (flyweight task) or AbstractBuild (with workspace)