Issue8

I added Database::remove_file() function to remove a file from the database, and added files_engine::delete_file() to remove the file from libtocc's file system, and the high level (front_end) function Manager::delete_file().
plus, Unit test of the function Manager::delete_file().
I didn't noticed this before!
This exception should be thrown inside the Database layer, specially because it's one of the Database errors.

And, please remove the trailing ``\n``. It's not going to always *print on the output*. And if so, client can add it itself. We don't want to force it.