add the mocha-snippets recipe.

This provides very common snippets when writing testcases in JavaScript
using Mocha (http://visionmedia.github.io/mocha/)

Here is the package repository of which I am the maintainer:

https://github.com/cowboyd/mocha-snippets.el

I've been using these snippets for over a year, and decided to extract a
subset of them so that I could share with my co-workers and the world.