Re-organize file structure

This series of patches reorganize Plinth source to look more like Python module

- 'plinth' is a python module
- Use setup.py for build and install instead of Makefile
- Run tests using setup.py
- Install to correct location in system
- Various smaller cleanups
- Run from system install using proper paths to /var and /usr
- Run properly from working directory
- Reorganize static folder and data files also

@NickDaly, @jvalleroy, @petterreinholdtsen, @fonfon, Please review the merge request if any of you find time.