Add square roots (\sqrt)

Summary:
Follow the TeXbook instructions on how to construct square roots. Using
makeCustomSizedDelim, this becomes nearly trivial.

Test Plan:
 - Make sure normal tests work
 - Make sure the new huxley test looks good, and other huxley tests haven't changed.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D12918

symbols.js

@@ -298,6 +298,11 @@ var symbols = {
             group: "bin",
             replace: "\u00d7"
         },
+        "\\surd": {
+            font: "main",
+            group: "textord",
+            replace: "\u221a"
+        },
         "(": {
             font: "main",
             group: "open"