Added student_id param when getting modules

Hi Whitmer, 

I'm offering you two fixes: 

* The first one to solve issue #9 
* The second one, to solve a bug concerning image_urls

The image_urls bug happens when a badge is already awarded and someone change the image_url in the badge configuration. When someone performs this change, the already awarded badges are not updated. So when you call "badge.badge_url" on an already awarded badge, it sends back the path to the old image - behavior that we don't want. 

As it really matter when a student wants to send his badge to backpack, I thought that we might check if badge_url is the same as the badge_config.settings["badge_url"] data, and if not, update it before sending the badge to backpack. 

If we don't do this and we remove the old image, an error is thrown when sending to backpack.