Remove 'before' attribute on imports and stable_topological_sort
BUG Support \\ as escape in run_job.substitute
run_job: New environment control commands (set/append_path/etc.)
Refactor imports: Add "on_import", remove PATH,HDIST_{CFLAGS,LDFLAGS}
run_job: Remove 'cwd' attributes in favor of 'chdir' command
run_job: ARTIFACT set to relevant artifact in on_import sections
Merge pull request #89 from hashdist/imports

Revamped import system, fixes #56