Correctly style fracs as inners

Summary:
Turn fracs into inners like they should be, instead of using the
previous dumb hack.

Test Plan: Make sure \dfrac still works

Reviewers: spicyj

Reviewed By: spicyj

Differential Revision: http://phabricator.benalpert.com/D50

MJLite.js

@@ -63,7 +63,7 @@ var buildGroup = function(group, prev) {
         var mid = makeSpan("mfracmid", makeSpan());
         var denom = makeSpan("mfracden", buildExpression(group.value.denom, denom));
 
-        return makeSpan("mord mfrac", [numer, mid, denom]);
+        return makeSpan("minner mfrac", [numer, mid, denom]);
     } else if (group.type === "color") {
         return makeSpan("mord " + group.value.color, buildExpression(group.value.value));
     } else if (group.type === "spacing") {

static/mjlite.css

@@ -80,6 +80,50 @@ big parens
     margin-left: 0.16667em;
 }
 
+.minner + .mord {
+    margin-left: 0.16667em;
+}
+
+.minner + .mbin {
+    margin-left: 0.22222em;
+}
+
+.minner + .mrel {
+    margin-left: 0.27778em;
+}
+
+.minner + .mopen {
+    margin-left: 0.16667em;
+}
+
+.minner + .mpunct {
+    margin-left: 0.16667em;
+}
+
+.minner + .minner {
+    margin-left: 0.16667em;
+}
+
+.mord + .minner {
+    margin-left: 0.16667em;
+}
+
+.mbin + .minner {
+    margin-left: 0.22222em;
+}
+
+.mrel + .minner {
+    margin-left: 0.27778em;
+}
+
+.mclose + .minner {
+    margin-left: 0.16667em;
+}
+
+.mpunct + .minner {
+    margin-left: 0.16667em;
+}
+
 .msub {
     vertical-align: bottom;
     font-size: 70%;
@@ -108,8 +152,6 @@ big parens
 .mfrac {
     display: inline-table;
     vertical-align: 0.66em;
-    padding-left: 0.16667em;
-    padding-right: 0.16667em;
 }
 
 .mfracnum, .mfracmid, .mfracden {