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