- add `kb.label` to retrieve (multi-lingual) labels associated to a term
- add support for querying direct classes, instances, subclasses, superclasses
  (via eg `kb.details`)
- significantly simplify the event subscription mechanism. Now, you simply
  pass a list of patterns with an optional `one_shot` parameter (false by
  default)
- better [API documentation](https://github.com/severin-lemaignan/minimalkb/blob/master/doc/api.md)
- remove `kb.check` as it was strictly equivalent to `kb.exist`
- fix bug where loaded ontologies would not be processed by the reasoner
- fix internal methods being mistakenly exposed in the API
- a few other minor bug fixes