add support to simply load xblock using the same host + sending extra data to student_view

This PR is to:

* Use jquery.xblock in the LMS. So, not having to specify the sessionsId and domain make it easier. We just need to specify {useCurrentHost: true}
* We can also add a data attribute in the options. Those will simply be send as get parameter to the student_view.

This needs to be added to the README, with an example usage.