Add Creative Commons Attribution 4.0 Open Source Licenses

Add Creative Commons Attributiion 4.0, Attribution-Sharealike 4.0, Attribution-NonCommercial 4.0, and Attribution-NonCommercial-Sharealike 4.0 licenses as shown [here](https://creativecommons.org/weblog/entry/41127). These licenses are important for people who want to license their works in general. For instance, if someone has a blog hosted on GitHub and wants to make it open-source, they may want to use a Creative Commons license so that their license is specific to the content as well as the code, as opposed to the code.