Updates to About, Practical Machine Learning, and Data Scientist's Toolbox

Hi there! Here's a description of my proposed changes:

* On the About page, I added a link to my blog.
* In Practical Machine Learning, I added a link to my post about confusion matrix terminology. This relates directly to week 1, lecture 6, "types of errors." Dr. Leek explains the terms most relevant to his biological example (disease vs no disease). My post covers these same terms and also other related terms that are commonly used in machine learning. I posted this link in the PML forums and a TA marked it as approved, so someone must think it's good :)
* In Toolbox, I added a download link for the videos from the Git video playlist. That is because when I posted these videos in the Toolbox forums, I have numerous times received requests for a download link by people who don't have access to YouTube (blocked by work, blocked in their country, etc). Anyway, the download link is to my Google Drive and I intend to keep the videos there permanently.
* In Toolbox, I added a link to my quick reference guide for Git commands. I wrote this guide because most Git guides either list every possible Git command or they are too general. My guide is specifically for beginners and presents commands grouped by workflow.
* In Toolbox, I added a link to my post about the relationship between Git and GitHub. That's because it became clear in the forums (from 4 sessions as a TA) that many students just don't understand how they work together. It is based around a quote from a student in the forums who explained it all quite well!
* In Toolbox, I added a link to my post about GitHub forks and the forking workflow. It repeats a tiny bit of the video content, but I have taught this material in person and have gotten really positive reactions from the diagrams, so I think it's a useful post for beginners.

If any of these links aren't what you're looking for, no worries! Just let me know what to revise or remove, and I'll gladly do another commit in this branch to update the pull request.

Thanks!