Add triggeredBy property to dialogs

[KevinDepuydt]

Added triggeredBy property to dialogs.

This property can be one of the following:

• nlu: the dialog has been triggered by the classification results
• dialog-manager: the dialog has been triggered by the dialog manager, this may happen when there is no classification results (default dialog triggered), when there is too much classification results (classification-disambiguation dialog triggered) or when a confirmation is required to continue a dialog (confirmation-dialog)
• postback: the dialog has been triggered by the click on a postback button
• -dialog: the dialog has been triggered by another dialog (using triggerNext or cancelPrevious or startNewConversation when the next dialog name is provided)

[yangeorget]

LGTM