Google Drive XBlock - Fake PR for review purposes, do not merge!

@martynjames Created this PR for the review. 

Btw for the future, it's easier to create this when the first commit is empty. Also, if we want to review a series of PRs at once instead of individual PRs, it would be easier to merge the individual PRs in a branch which isn't master, and then create a PR from that branch into master when it's ready -- this way we don't need "fake" PRs at all, they represent the actual review workflow.
it appears as though a user can include either an iframe or an image.  Thanks for adding support for the title attribute on the iframe.  However, the title attribute is inappropriate for an image.  What you want, is the alt attribute.  That said, the display_name is not going to be a suitable value for the alt attribute.  We really need a separate mechanism to allow a user to include an alternative description for an included image.