Update metrics using TeX metrics instead of TTF metrics

Summary:
Instead of using the metrics from our generated TTF files, use the fonts that
TeX ships with. Pull the mapping out of the MathJax-dev repo from makeFF to get
the correct mapping of metrics to font characters, and use our own tfm reader to
extract metrics out of the tfm files into a useable format. Add a README and
Makefile rule to make this process easier in the future.

Also remove the silly 0.05em we put on supsubs because our italic correction
works now.

Test Plan:
- Run huxley tests, see that changes are because of font metric changes.
- See that the extension piece of `\bigl |` now extends above the top, as it is
  supposed to.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D12867