DON'T MERGE -- Displaying math notation demo

I hacked together a way of displaying math notation. You must be using R Studio for it to work in it's current form. Also, I'm getting a couple of WARNINGs on `R CMD check` that I need to resolve, but you'll get the basic idea.

Build from this `math_notation` branch, then run the following:

```
library(swirl)
install_from_swirl("Exploring_Math_Notation")
swirl()
```

Let me know what you think.