Version 2.0.0

The big changes are failed tasks (contributed by @unode).

Also, environmental variables are now always checked and creating a file called __jug_please_stop.txt will stop a jug execute run in a clean way.

Full ChangeLog

• jug.backend.base_store has 1 new method listlocks
• jug.backend.base_lock has 2 new methods fail and is_failed
• Add 'jug execute --keep-failed' to preserve locks on failing tasks.
• Add 'jug cleanup --failed-only' to remove locks from failed tasks
• 'jug status' and 'jug graph' now display failed tasks
• Check environmental exit variables by default (suggested by Renato Alves, issue #66)
• Fix 'jug sleep-until' in the presence of barrier() (issue #71)