RLS 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)

ChangeLog

@@ -1,4 +1,4 @@
-version 2.0.0 Sat Feb 1 2020 by luispedro
+version 2.0.0 Fri Feb 21 2020 by luispedro
 	* 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.

README.rst

@@ -134,7 +134,7 @@ the `value` function::
 What's New
 ----------
 
-version **2.0.0** (Sat Feb 1 2020)
+version **2.0.0** (Fri Feb 21 2020)
 - 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.

docs/source/history.rst

@@ -2,7 +2,7 @@
 History
 =======
 
-version **2.0.0** (Sat Feb 1 2020)
+version **2.0.0** (Fri Feb 21 2020)
 - 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.

jug/jug_version.py

@@ -1,4 +1,4 @@
-__version__ = '2.0.0rc0'
+__version__ = '2.0.0'
 CITATION = '''\
     Coelho, L.P., (2017). Jug: Software for Parallel Reproducible Computation in
     Python. Journal of Open Research Software. 5(1), p.30.