I didn't attach a license to this as: a) the front-end is just a very simple UI put on top of attacklab.net's showdown.js. Unfortunately, attacklab seems to be dead now but the showdown code has a BSD license, see: http://static.joncom.be/screen/script/showdown.js. b) the back-end simply opens and saves text files, I didn't think anyone would want to see that.

In answer to your question, attacklab did the hard work and yes there is an open source license attached to his/her work.