Spruced up ch02.ipynb a bit by adding comments to most of the inputs

[lkissin2]

No description provided.

[scopatz]

Thanks @lkissin2! What are the substantive changes here? It is hard to see from the diffs...

[lkissin2]

I added comments like

# bad idea to start a variable name with a number

or

# good way to put a number in a variable name is to have it anywhere but the 0th character

I also wrote notes in chapters 3 and 4 with Markdown, but I haven't PR'd those yet. Is there a certain style guide I should be following?

Thanks!