Add stack implementation in JavaScript

[richardartoul]

Hey guys,

I added a Javascript implementation for stacks. I have a question before merging:

The JavaScript testing system seems to run all the code samples through Node (after injecting the assert library) whereas the Python testing system runs the separate unit test files. How do you want me to handle testing the JavaScript stuff? I think it'll look weird if we have assert statements in all the JavaScript code samples, but not in the Python code samples. Its not the end of the world though. Would you rather I continue doing that (in which case I'll add tests to this pull request) or that I set up some generic JavaScript testing system we can use?

[ozan]

I actually like the asserts when they serve an illustrative purpose for the reader. I haven't yet encountered a situation where I wanted JavaScript tests that were not also illustrative for the reader, but only at that point would I set up some other js testing system. Up to you though.

Regarding the CI fail, you'll need to add the gross "coding" magic comment # -*- coding: utf-8 -*-