Merge pull request #71 from larsks/bug/messageid

use message-id instead of str(message) in logs
Merge pull request #75 from larsks/feature/add_register_filter

add missing register_filter function
qualify list-id when generating tags

Using an unqualified list-id as a tag name allows anyone sending you
mail to set arbitrary tags on messages.  This patche puts all list tags
in a 'list/' namespace.
tags_blacklist is a list

treat tags_blacklist like tags: read it from config with get_list() and
specify it as a list in class definitions.
lowercase generated tag names

Rather than using header values verbatim, pull everything to
lower case when generating tag names.
give filters a self.log attribute for better logging

This patch modifies BaseFilter.py to provide filter classes
with their own logger named after the name of the class.  This makes it
easier to identify which filter modules are generating log messages (and
would ultimately allow one to apply different loglevels, etc., to
different classes).