Random discovery: make intial math rendering twice faster :-)

On a math-heavy 100K doc, speeds up "All math rendered" from 38s to 18s!

#68, cben/CodeMirror-MathJax#14

I was doing early hookMath() and postponing renderAllMath() on the theory
"Queuing this allows text to appear before math."
But doing hookMath() before firepad load already renders all math,
and renderAllMath() does all the work again because there is no caching
(cben/CodeMirror-MathJax#15).

mathdown.js

@@ -182,8 +182,6 @@ function setupEditor(editor) {
       updateTitle();
     }
   });
-
-  CodeMirror.hookMath(editor, MathJax);
 }
 
 // Firepad
@@ -219,6 +217,7 @@ function setupFirepad(editor, firepad) {
 
     // Queuing this allows text to appear before math.
     MathJax.Hub.Queue(function() {
+      CodeMirror.hookMath(editor, MathJax);
       editor.renderAllMath();
     });
   });