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).
Merge pull request #82 from larsks/feature/per_class_log

give filters a self.log attribute for better logging