Novice-level lesson on branching in Git

Adding a lesson on branching for novice git users. Kept it as simple as could be, introducing the basic branching commands only. I am on the fence about how to resolve the lesson so I would love to have feedback on the following-

**Update 5/1: Upon discussing the options went with teaching a PR on GitHub proper for the resolution to the lesson.** 

*Resolving the lesson:*
In line 155, I introduce merging. This will be problematic in the command line with the fast-forward vs. regular merge issue. To address this there are a few options.
* **Pull request-** If we introduce a pull request on GitHub, we can show them `git log --oneline --graph` so they can have a nice visual at the end showing their history in the command line. I personally think this is the best option. 
* **Explaining fast-forward vs. regular merge-** A run for the hills option, imho, it might be a little much to throw on them at this point.
* **Have them commit again to the master before merging-** A run for the mountains approach, they add a dummy commit in their workflow to retain branching history.

Any feedback on the rest of the lesson is welcome, tried to keep it consistent with Greg's lessons using the Monster Mash story line.
Agreed with branching after open science; people are pretty fried by the end of conflicts.