Fix Waypoint in Firefox

The span that Waypoint renders to the page is rendered differently in
Firefox than Chrome. In Chrome, it has a height of 0, whereas in Firefox
it has a height that matches its font-size. This caused height
calculations in our specs to be off by 16px in Firefox. Giving this
element a font-size of 0 resolves this issue.

Author: lencioni

src/waypoint.js

@@ -138,7 +138,7 @@ var Waypoint = React.createClass({
   render: function() {
     // We need an element that we can locate in the DOM to determine where it is
     // rendered relative to the top of its context.
-    return React.createElement('span');
+    return React.createElement('span', { style: { fontSize: 0 } });
   }
 });