Opalstack

- Adds `apps.mark_installed`, a method for an app install script to mark its installation complete

- Adds Usage API support
- Usage data now accessible via `opalapi.usage.webusage_latest()` and `opalapi.usage.mailusage_latest()`

This release of opalstack-python brings the following changes:

- Bug fixes in [clone_wp_site.py](https://github.com/opalstack/opalstack-python/blob/master/examples/clone_wp_site.py) example script.
- Addition of `opalstack.VERSION` and `opalstack.__version__` module-level constants

Many thanks to cvn for reporting issues and submitting flawless PRs. Cheers!

Initial public release