Fixed ImportError when running as non-daemonized python package

* running plinth uninstalled via `python -m plinth` works now. The problem was cherrypy's os.chdir() followed by package-internal imports. Include these imports in `__init__.py` to fix the problem, see [this StackOverflow question](http://stackoverflow.com/questions/25641003/how-to-debug-importerror-with-sys-path-being-correct) for details
* renamed `get_group` to `get_or_create_group` to be closer to [django method names](https://docs.djangoproject.com/en/dev/ref/models/querysets/#get-or-create)
* minor string formatting improvement