Undo/Redo—initial implementation

WYMeditor's undo is, in some cases, not accurate.
So much more work, that would be. The `object-history` module has its own tests, so I wrote these with the purpose of testing that the various different actions are registered. If one reads along this test, it should make sense to him/her, I think.

The question is, what occurs when something goes wrong and some assertions fail, within this test. Would it be easier to debug if it were separate, focused, test cases? I'm not sure it is worth it.

Do you think it is worth it?