Eliminate implicit relative imports

Convert to either absolute imports or explicit relative imports
Use isinstance() instead of callable()
Require and use Python3
Update dependencies list in setup.py and INSTALL
Make actions work with Python3
Remove outdated and not-working Santiago module
call django.setup before syncdb for django 1.7 compatibility
Remove box key related code from first boot

- Box key is not being used currently
Move enabled modules list to configuration directory
Convert plinth/modules to a namespace package (PEP 420)

- This will allow packages outside plinth to add modules to plinth
Update documentation with Python3 dependency information
Load modules using explict paths instead of config file name
Turn modules into proper Django applications
Make setup.py sdist command work properly
Fix issue with symlinks during recursive copy during install
Fix issue with plinth module not getting installed
Update documentation with setuptools dependency, python3 (James Valleroy)
Release 0.4