Support \atop.

Add Jasmine test and update screenshotter test.

test/katex-spec.js

@@ -606,6 +606,15 @@ describe("A frac parser", function() {
         expect(tfracParse.value.numer).toBeDefined();
         expect(tfracParse.value.denom).toBeDefined();
     });
+
+    it("should parse atop", function() {
+        var parse = getParsed("x \\atop y")[0];
+
+        expect(parse.type).toEqual("genfrac");
+        expect(parse.value.numer).toBeDefined();
+        expect(parse.value.denom).toBeDefined();
+        expect(parse.value.hasBarLine).toEqual(false);
+    });
 });
 
 describe("An over parser", function() {

test/screenshotter/ss_data.yaml

@@ -62,7 +62,7 @@ DisplayStyle: |
     {\displaystyle\sqrt{x}}{\sqrt{x}}
     {\displaystyle \frac12}{\frac12}{\displaystyle x^1_2}{x^1_2}
 Exponents: a^{a^a_a}_{a^a_a}
-FractionTest: \dfrac{a}{b}\frac{a}{b}\tfrac{a}{b}\;-\dfrac12\;1\tfrac12
+FractionTest: \dfrac{a}{b}\frac{a}{b}\tfrac{a}{b}\;-\dfrac12\;1\tfrac12\;{1 \atop 2}
 Functions: \sin\cos\tan\ln\log
 GreekLetters: \alpha\beta\gamma\omega
 KaTeX: \KaTeX