Parse single superscripts and subscripts

Summary:
Add the ability to parse lone `^x` and `_y`, etc. This basically just
involves more checking of cases in the increasingly ugly `parseAtom` function.
Also, now we manually check for the cases of double superscripts and
subscripts.

Test Plan: Make sure the tests pass. Make sure things like `^x` and `_y` parse.

Reviewers: emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D3095

katex.js

@@ -27,6 +27,10 @@ var makeSpan = function(className, children) {
 };
 
 var buildGroup = function(style, color, group, prev) {
+    if (!group) {
+        return makeSpan();
+    }
+
     if (group.type === "mathord") {
         return makeSpan("mord" + color, [mathit(group.value)]);
     } else if (group.type === "textord") {