reformat Robin's code

Tabs are evil. Better not indent code with tabs. Also apply fill-column
70 so that code is more readable on a wider range of text editors.

Robin: if you have any objection, feel free to discuss with me. We can
easily revert the changes by running 'git revert'.
use 'n' instead of 'N' for dimension of...

projective space.

N is later used for the length of cycles. Must be renamed to avoid name
clash.
add Zhiming's comments regarding the N=4 section

To summarize, I think we need more precise definitions (since there are
crucial paradigm shifts) and a little bit more math about the rational
points (due to their central role).
Merge pull request #4 from zmwangx/master

Presentation 1: improve section 1 and 2, and comment on section 2