Remove form handlers on destroy:composer

[KProskurdin]

If the textarea is in a form, handlers for submit and reset events are created. But after delete of editor those handlers are still present for the form. And if you try to submit form after editor is removed error will be risen.
Stack trace of error is like this:

getDocument
parse
setValue
fromComposerToTextarea
sync
(anonymous function)

Anonymous function is from here

wysihtml.dom.observe(form, "submit", function () {
    that.sync(true);
});

This pull request is to solve this issue.