convenience method for delegating jobs to subtasks

Change-Id: Icfb1a6aa136954c962c0621bba70fcf813811c0f
Reviewed-on: https://gerrit.spotify.net/gerrit/45426
TC-Compile-And-Test: Team City <teamcity@spotify.com>
Reviewed-by: Elias Freider <freider@spotify.com>
fixed bug with LocalTarget and is_tmp where files were created at instantiation

https://github.com/spotify/luigi/issues/29

Change-Id: Ibc6ad08a1084605c549ed00be90d84d8d3c700de
Reviewed-on: https://gerrit.spotify.net/gerrit/46388
TC-Compile-And-Test: Team City <teamcity@spotify.com>
Reviewed-by: Erik Bernhardsson <erikbern@spotify.com>