Fixed ImportError when running as non-daemonized python package

Created by: fonfon

• 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 for details
• renamed get_group to get_or_create_group to be closer to django method names
• minor string formatting improvement
• fixed static dir of plinth modules