Features
* Add function `validation.valid_int()`
Fixes and enhancements
* Add constants to `juju` module
* Juju bootstrap will print time as int
* Add `makedirs` argument to some methods of the objects of `serialization`
* Add user argument to function `subprocess.cmd`
* Add path argument to `subprocess.make`
* Add extra_args (list) to function `subprocess.rsync`
* Fix `juju`, `serialization`, `subprocess` modules, update tests
* Function subprocess.cmd : Handle `logging.Logger` as log, improve docstring, add retry loop
* Upgrade `relation_` methods