Expand role detection to support Blackboard's LTI implementation

I found that Blackboard prefers to send the Instructor role within the
"urn:lti:role:ims/lis/" URN. The support was already here, I just
extended the detected roles. Hooray for Blackboard support in
BigBlueButton!
Update bbb-lti README to note trailing slash issue

Save yourself hours of LTI debugging! Copying/pasting the URL bbb-conf
--salt brings along a trailing slash, which bbb-lti does NOT
appreciate. :)
Allow LTI to pass custom welcome messages
Adding recording support to the LTI middleware Grails app

When configuring an LTI consumer, pass [custom_]record=true to indicate
whether the session should be recorded.
Merge pull request #94 from kishba/master

Verified changes that work with Blackboard's LTI support.  Thanks!