Make `rotate-windows` aware of dedicated windows.

Hello, and thanks for your new "What the .emacs.d!?" blog, in which I learned tons of interesting stuff.

I was very interested by your `rotate-windows` function, but it didn't always work in my case since I tend to use dedicated windows (for example for compilation or help buffers).
In case you're interested, here is a version which leaves dedicated windows untouched.

Thanks again,
   François