HTML Comments cause an exception

The error triggers on [line 552](https://github.com/fiduswriter/diffDOM/blob/gh-pages/diffDOM.js#L552) because `t1.attributes` is undefined.

The fix might be as simple as adding a check like 

``` javascript
t1.attributes ? slice.call(...) : []
```

(and same for `t2` on next line), but I haven't tested it yet.
