This is necessary to properly implement the LTI outcome passback
spec. It was not included previously because it was causing problems
with Canvas, but Blackboard will not accept outcomes unless they are
properly signed.

The requests_oauthlib doesn't support the body hash spec out of the
box, so BodyHashClient needed to be made. Fortunately, it's a pretty
simple spec:

https://oauth.googlecode.com/svn/spec/ext/body_hash/1.0/oauth-bodyhash.html