allow custom file headers

You can now place a file called .kdev_python_header in any project directory;
its contents will be placed at the beginng of every python file in that project
internally. So, if you need the python3 print syntax, you can now emulate this by
putting a .kdev_python_header file containing
from future import print_function
in your project directory. It's a really ugly hack, but it works.