Uploading updated and improved version of GoogleDocViewer

GoogleDocViewer is an extension whose code is only available at
https://www.mediawiki.org/wiki/Extension:Googledocviewer
I downloaded the code, updated and improved it, and I'm now
uploading it to Gerrit.

Change-Id: I63a79aea78835fae58ca041131f4fcb192eb5cae