Allow to unannotate information

Closes #336 

Adds ability to unannotate in purpose and save such changes through the `TreebankPersister`. Check to comments in the persister's `updateWord()` function.

A minor fix for `morph` is included as well: `morph.unsetState()` did not participating in the `state` eventing, which meant that the `saver` did not react to these changes and `history` wasn't able to record such an event.