Creative Commons Licensing for edX

This PR adds Creative Commons Licensing to the Open edX Platform.

Review tickets
T&L https://openedx.atlassian.net/browse/TNL-195
UX https://openedx.atlassian.net/browse/UX-948
Rather than calling `validate` here, you want to have `validate` defined on the model, so that [Backbone will call it automatically](http://backbonejs.org/#Model-validate). You also don't have to worry about checking if the license is different -- if you try to set a value that is identical to the existing value, it's a no-op and the `changed` event will not be fired (as far as I'm aware).