Remove annoying debug log message
Move SQLite initialisation to db module
Small simplification
Rename some functions

This should make some functions clearer and marks internal functions
with a leading underscore.
Split functionality into Link.get_or_create()

This splits some common functionality from Link._get_child() and
Crawler.get_link() to the new Link.get_or_create() function.
Remove duplicate column definition
Provide a CSV file plugin

This plugin generates a simple CSV file with all the URLs in the system
and some basic information about them.