Added ability to discard the pending change set from the session.

Whilst working with the lib I've found it quite often useful to be able to discard pending changes as part of a rescue cleanup if something happens in the app between the creation of the record and publication.

The API allows for this https://help.dynect.net/delete-zone-change-set-api/ so I have added 'discard_change_set' as a feature of the lib.