Fix issue with installing to a different root with --root=
Clean documentation directory when 'setup.py clean' is run
Relicense __main__.py to AGPLv3+

- There is no point in keeping this one single file as with different
  license from the rest of the package.  This change simplifies things.

- The file has been almost entirely been rewritten for Django, module
  loading, CherryPy static directories etc.

- This change will be pushed with the consent of new authors: fonfon and
  Me.
Update copyright information

More authors have contributed in a significant manner given them
copyright for thier contributions.  Hence changed Plinth copyright to
'Plinth Authors'.  Full list of authors can be obtained from the Git
revision log.
Remove copyright information in footer for cleaner apperance
Remove #! from actions.py as it is not meant to run independently
Explicitly write license headers for each source file

The package license (AGPL3+) implicitly indicates the license of each
file.  However, it is desirable to have license headers in each file.
This is the case for many prominent projects like GNU project, Mozilla
etc.