Added a zen helper to IEx

I was reading "Programming Elixir", and I was really inspired by Dave Thomas when he wrote:

* Object-Orientation is not the only way to design code.
* Functional programming need not be complex or mathematical.
* The foundations of programming are not assignment, if statements, and loops.
* Concurrency does not need locks, semaphores, monitors, and the like.
* Processes are not necessarily expensive resources.
* Metaprogramming is not just something tacked onto a language.
* Even if it is work, programming should be fun.

  -- Dave Thomas in "Programming Elixir"

This reminded me a lot of what happens when you write "import this" in Python. Hence, I added an IEx helper that outputs this message when you type zen.