Make imports look like includes

Imports work like includes so they should use the relevant group-name.

I also renamed pythonPreCondit to pythonImport since it's specific to
that type of "preprocessor" statement.
Update CHANGES.txt
Add new option 'python_highlight_file_headers_as_comments'

The option is disabled by default and when enabled highlight shebang and
coding file headers as comments instead of special characters.