Add jQuery amend plugin which smoothly alters HTML

This is a version of $("...").html() which syncs over the HTML and avoids needlessly replacing nodes or elements.
Also, I think that sync is never called with depth > 0 and either node having a type other than 1 or 3, because of the explicit skipping of those types before the recursive call.  So maybe this condition could be simplified to

```
if (output.nodeType != input.nodeType) ...
```