Verified changes that work with Blackboard's LTI support

With a little bit of tinkering, I've been able to get the bbb-lti grails app to work with my institution's non-production Blackboard installation. Blackboard's LTI connection sends roles under the plain "role" URN as opposed to the "instrole" URN.

I also had some difficulty with configuring the BigBlueButton URL, so I'm proposing an slightly modified README to help future sysadmins get up and running.

Oh, this is my first-ever commit to an open source project. I'm stoked! :smiley: