Added

• added Dockerfile for rasa_core_sdk
• add active_form and latest_action_name properties to Tracker
• add FormAction.slot_mapping() method to specify the mapping between
  user input and requested slot in the form
• add helper methods FormAction.from_entity(...),
  FormAction.from_intent(...) and FormAction.from_text(...)
• add FormAction.validate(...) method to validate user input

Changed

• FormAction class was completely refactored
• required_fields() is changed to required_slots(tracker)
• moved FormAction.get_other_slots(...) functionality to
  FormAction.extract_other_slots(...)
• moved FormAction.get_requested_slot(...) functionality to
  FormAction.extract_requested_slot(...)
• logic of requesting next slot can be customized in
  FormAction.request_next_slot(...) method

Removed

• FormField class and its subclasses

Fixed