Get rid of the lookahead-y dots

Summary:
The `\dots` and `\dots{c,o,b,i,m}` commands do more than just show characters,
they add extra space and `\dots` also looks ahead to determine what kind of
environment it is in (see [this math.sx post]
(http://tex.stackexchange.com/questions/122491/difference-of-the-dots)). We
can't support that yet, so remove them.

Also, add comments to the extract_ttf script to show what caracters we are
extracting metrics for.

Test Plan: - Make sure tests work

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D13151