Add support for \overline

Summary:
Follow the instructions in the TeX book for drawing \overlines. This uses the
same code as fractions to produce the bars. Also added the ability to cramp
styles (i.e. T -> T' and T' -> T').

Test Plan:
 - Look at `\overline{x}`, `\overline{\dfrac{x}{y}+z}`, and
   `\blue{\overline{x}}` to make sure they look good.
 - Make sure the tests work
 - Make sure the huxley tests look good (Not here yet :( )

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D11604

test/huxley/Huxleyfile.json

@@ -99,5 +99,11 @@
         "name": "DelimiterSizing",
         "screenSize": [1024, 768],
         "url": "http://localhost:7936/test/huxley/test.html?m=\\bigl\\uparrow\\Bigl\\downarrow\\biggl\\updownarrow\\Biggl\\Uparrow\\Biggr\\Downarrow\\biggr\\langle\\Bigr\\}\\bigr\\rfloor"
+    },
+
+    {
+        "name": "Overline",
+        "screenSize": [1024, 768],
+        "url": "http://localhost:7936/test/huxley/test.html?m=\\overline{x}\\overline{x}\\overline{x^{x^{x^x}}} \\blue\\overline{y}"
     }
 ]

test/huxley/Overline.hux/record.json

@@ -0,0 +1,5 @@
+[
+  {
+    "action": "screenshot"
+  }
+]

test/katex-tests.js

@@ -667,3 +667,19 @@ describe("A delimiter sizing parser", function() {
         expect(bigParse.value.size).toEqual(4);
     });
 });
+
+describe("An overline parser", function() {
+    var overline = "\\overline{x}";
+
+    it("should not fail", function() {
+        expect(function() {
+            parseTree(overline);
+        }).not.toThrow();
+    });
+
+    it("should produce an overline", function() {
+        var parse = parseTree(overline)[0];
+
+        expect(parse.type).toMatch("overline");
+    });
+});