Add code for generating HTML

Test Plan: Ran unit tests. Looked at `\blue{\displaystyle \left(\dfrac{a^\sigma}{\sin \theta}\right\Updownarrow \intop_{1/2}^{z^z} \sum_{i=0}^\infty x \,dx}` in Chrome and saw the future in my eyes.

Reviewers: emily

Reviewed By: emily

Subscribers: jessie

Differential Revision: http://phabricator.khanacademy.org/D13154

test/katex-spec.js

@@ -1,3 +1,4 @@
+var katex = require("../katex");
 var buildTree = require("../buildTree");
 var parseTree = require("../parseTree");
 var ParseError = require("../ParseError");
@@ -986,3 +987,14 @@ describe("A bin builder", function() {
         expect(getBuilt("\\blue{x+}+y")[1].classes).toContain("mord");
     });
 });
+
+describe("A markup generator", function() {
+    it("marks trees up", function() {
+        // Just a few quick sanity checks here...
+        var markup = katex.renderToString("\\sigma^2");
+        expect(markup.indexOf("<span")).toBe(0);
+        expect(markup).toContain("\u03c3");  // sigma
+        expect(markup).toContain("margin-right");
+        expect(markup).not.toContain("marginRight");
+    });
+});