Filter: Create classe

This is the base class for all Filters. It supports some basic operations like:
 * Determining the kind of the filter
 * Passing debug/warning/error messages
 * Running the filter
Filter: Add documentation and self._()
Filter: Implement message sending functions
tests: Add filter package
FilterTest: Add empty test
FilterTest: Add TestFilter Filter class
FilterTest: Test Filter queues and kind

This implicitly tests the call of tear_up and tear_down.
It also assures that messages are packed into a log message correctly.
Merge pull request #44 from sils1297/wip/filter

Filter: Add class and tests