check if _gat exists before including ga.js

This is supposed to allow users to use dc.js with gas. 
They should include the dc.js file synchronously before the _gas code 
snippet.

src/wrapup.js

@@ -7,7 +7,7 @@ while (_gas._queue.length > 0) {
 }
 
 // Import ga.js
-if (_gaq && _gaq.length >= 0) {
+if (typeof window._gat === 'undefined') {
     (function () {
         var ga = document.createElement('script');
         ga.type = 'text/javascript';