add missing register_filter function

This patch introduces a decorator, register_filter, that is used to
register a new filter class with afew:

   from afew.FilterRegistry import register_filter

    @register_filter
    class MySpiffyFilter(Filter):
        message = 'This filter does amazing things.'
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).