Simplification of replaceWith method

Not for 1.10.0 or 2.0.1.
It's smaller:
<pre>
   raw     gz Compared to master @ 997da31121b9d084ccba05a9bb1e258c8c8faaf0
  -440   -141 dist/jquery.js
   -78    -37 dist/jquery.min.js
</pre>
and faster: http://jsperf.com/simplified-replacewith

Although this code (as code in master) fails with self-replacement <a href="http://jsfiddle.net/gejGm/">cases</a>. But i suppose these are very edge stuff giving that jQuery lived with them since the beginning.

/cc @gibson042