Write errors to log file

The error log file is specified within the configuration file, like
with Apache.  You can also now have blank lines in the configuration
file.
Remove vestigial debug-mode code

This has been made obsolete by the newer code which lets the user
specify whether messages retrieved over POP3 should be deleted or
not afterwards, via the configuration file.
Update note of what I should do next

Error logging wise.
Add download log file

Make output more terse, even non-existent if all goes well.

Fix bug causing erroneous error log file to be created.
Add more exception handling with the error log
Remove obsolete comments
Ignore vi's .swp files in the Git repository
Fix logging
Use the GNU GPL V3