ie8 Fix

IE8 runs "onreadystatechange" twice, one for "loading" and another one for "loaded", by checking "this.readyState" we can check if it's already loaded but on chrome, this.readyState, is set as undefined, so the best way is to check if it isn't loading.
teste on Safari(6.0.3),chrome(31.0.1650.57 m), IE8(8.0.6001.18702), Opera(18.0.1284.49 )

another possible fix is by setting the callback function on the js itself.
Merge pull request #2 from jlvcm/patch-1

ie8 Fix - thank you