Add more context to code snippets in the todo list tutorial.

Added the filename being edited to the top of each snippet as well as
the object to place the method being added/edited if relevant.

As a noob to the language and framework, I found having to guess where some of the snippets
were going to be an unnecessary obstacle. This was mostly only the case in the section where 
you implement `Task`'s SQL functions, but I did the same with everything to be consistent.