ntex-mqtt v0.9.0-b.0

Changes

• Renamed Topic into TopicFilter, TopicError into TopicFilterError
• Changes to topic filter validation: levels starting with $ are allowed at any level and are recognized as system
  only at first position
• Changes to topic matching logic: when topic filter is matched against another topic filter via TopicFilter.match_filter(),
  left hand side topic filter must be strict superset of all topics allowed with topic filter on right hand side
• Changes to topic matching logic: having +/# in the end of topic filter does not wrongly recover failed match on + level
• Validation is now part of TopicFilter instantiation, e.g. it is impossible to create non-validated topic filter from
  set of Levels.
• Level API is removed completely as level itself is not a valuable concept.