Improvements

[\#130](https://github.com/rasahq/rasa/issues/130): Copied over instance methods `last_executed_action_has()`, `get_last_event_for()` and

:   `applied_events` from the Rasa `DialogueStateTracker` class to the SDK `Tracker` class.

• #145: Add poetry for dependency management and reduce the size of Docker image.

Miscellaneous internal changes

• #148, #149