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.'
Merge pull request #75 from larsks/feature/add_register_filter

add missing register_filter function