Fix typos

Nice paper! As I was reading it, I thought I'd clean up some of the more obvious typos that I saw.

I also tried to call out obvious todos by prefixing them with **TODO:**, that way when you have to look through the code to see what still needs your attention all you have to do is grep for TODO:

With this, I got up to the start of ## Test Driven Implementation. Keep up the good work!