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

metric_parse.rb

@@ -39,7 +39,7 @@ end
 font_dir = File.join(File.dirname(__FILE__), 'static/fonts/')
 metrics = {}
 
-%w[main-regular math-italic].each do |face|
+%w[main-regular math-italic ams-regular].each do |face|
   metrics[face] = metrics_for_file(File.join(font_dir, 'katex_%s.ttf' % face))
 end