Comment that clarify what we are doing.
Add a print_debug function

This function may only print thing when the
ENABLE_DEBUG flag is TRUE. Readability is
better and there is less tests in the code.
Add a function to print database errors

Database errors are considered as fatal for now. It
may be modified in the future.
Create all the tables into the database file.

I renamed some fields as they were reserved words such
as 'group' and 'order'. The documentation in file
README.md reflect this change.