Undo/Redo

WYMeditor's undo is, in some cases, not accurate.
I'm not yet sure, but I'm kind of guessing that this is the right thing to do. `_exec` is defined per browser engine. It generally passes on to the native `execCommand` function and the whole thing is probably synchronous. So, whatever that did, it is probably a modification to the document and should be deserving of a `registerChange()`.