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

test/huxley/Huxleyfile.json

@@ -129,5 +129,11 @@
         "name": "Rule",
         "screenSize": [1024, 768],
         "url": "http://localhost:7936/test/huxley/test.html?m=\\rule{1em}{0.5em}\\rule{1ex}{2ex}\\rule{1em}{1ex}\\rule{1em}{0.431ex}"
+    },
+
+    {
+        "name": "PrimeSpacing",
+        "screenSize": [1024, 768],
+        "url": "http://localhost:7936/test/huxley/test.html?m=f'+f_2'+f^{f'}"
     }
 ]

test/huxley/PrimeSpacing.hux/record.json

@@ -0,0 +1,5 @@
+[
+  {
+    "action": "screenshot"
+  }
+]