Allow the use of partner.coursera.org websites for Coursera submission

The submission URL is currently hardcoded as http://class.coursera.edu/...

However, classes set up on Coursera partner internal sites (such as mine) can have addresses such as princeton.coursera.edu, which means one can't submit Coursera assignments to them. With this change one can add an (optional) "Partner" attribute to a lesson's metadata, such that it ends with:

    Type: Coursera, Version: 1.0, Partner: princeton}

and it would know to send the submission to http://princeton.coursera.edu/...

I couldn't think of a way this could be done besides adding an additional metadata field.