Add some AMS characters

Summary:
\nleq and \ngeq are AMS characters, so add support for parsing and
showing those. Also, add font metrics from the ams_regular font.

Test Plan:
View `\ngeq \geq`, make sure that they look the same but with a
slash through `\ngeq`. View `a \ngeq b`, make sure there's the right spacing
for a rel.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3547

Parser.js

@@ -265,11 +265,13 @@ var copyFuncs = {
         "\\leftarrow",
         "\\leq",
         "\\neq",
-        "\\ngeq",
-        "\\nleq",
         "\\rightarrow",
         "\\to"
     ],
+    "amsrel": [
+        "\\ngeq",
+        "\\nleq"
+    ],
     "spacing": [
         "\\ ",
         "\\,",