Jug

* Add return_value argument to jug_execute
* Add exit_env_vars

* Fix bug with `invalidate()` in the shell

* Add 'graph' subcommand - Generates a graph of tasks
* 'jug execute --keep-going' now ends with non-zero exit code in case of failures
* Fix bug with cleanup in dict_store not providing the number of removed records
* Add 'jug cleanup --keep-locks' to remove obsolete results without affecting locks

* Add 'demo' subcommand
* Add is_jug_running() function
* Fix bug in finding config files
* Improved --debug mode: check for unsupported recursive task creation
* Add invalidate() to shell environment
* Use ~/.config/jug/jugrc as configuration file
* Add experimental support for extensible commands, use
~/.config/jug/jug_user_commands.py
* jugrc: execute_wait_cycle_time_secs is now execute_wait_cycle_time
* Expose sync_move in jug.utils

* Fix bug with writing very large objects to disk
* Smarter handling of --aggressive-unload (do not unload what will be
immediately necessary)
* Work around corner case in `jug shell` command
* Add test-jug subcommand
* Add return_tuple decorator

* Update `shell` subcommand to IPython 5
* Use ~/.config/jugrc as configuration file
* Cleanup usage string
* Use `bottle` instead of `web.py` for webstatus subcommand
* Add `jug_execute` function
* Add timing functionality