Added mutate.go

Hello, thanks for your terrific goquery library!  I wanted to use it in a templating system I'm building, but I had to extend it a bit so that it could manipulate the parse tree.  It's by no means a complete clone of the jQuery manipulation API, but it's a few of the good ones, and it's more than there had been!

If you'd like to see it in action check out github.com/dunmatt/gats

Have a good one!