Bug 1043682 - Caret positioning glitch after deleting a saved draft

Since we remove and then add ```<br>``` maybe we can avoid DOM operations and operate on selection only? Like this:
```
var range = document.createRange(),
	selection = window.getSelection();

if (dom.message.lastChild.tagName === 'BR') {
	range.setStartBefore(dom.message.lastChild);
} else {
	range.setStartAfter(dom.message.lastChild);
}

selection.removeAllRanges();
selection.addRange(range);
```
Regarding to unit tests, maybe we can just analyze offsets of selection range in [1] instead of mocking? Not sure though if it can cause intermittents, but worth checking
```
window.getSelection().getRangeAt(0).start\endOffset
```
[1] https://github.com/mozilla-b2g/gaia/blob/c5a673a737b48c3a2b34c45cd59194364aa19cf9/apps/sms/test/unit/compose_test.js#L524

What do you think about all of this? :)