Add "4Clojure.com" to the tutorial section

I thought, it would be nice to have this site too. These are Koan-styled exercises, which teach you a bit about Clojure, but you should have own knowledge (for the complexer problems). The problems are also ranked by difficulty.

Note: Since i really didnt now how to sort the number "4" in, i made it like in ASCII (numbers before letters).