Include warning if version of documentation is not x.y.z - fixes 560

Fixes #560.

This strategy may need a slight tweak as part of #561, depending how we keep the version in `conf.py` stable on release branches.
The use of `document.write` seems a bit old fashioned, and it's easy to make mistakes in the written HTML string, but as this SO http://stackoverflow.com/a/802894 answer says, it is very browser compatible and it does the job here. 

I guess I might have chosen to show or hide the warning box by dynamically adding a style attribute or a CSS class to the surrounding div....maybe using jquery. But that's a matter of taste.