Detect pdfjs version

Add utilities for PDF.js version-based decisions,
and add a marker class for ironing out a specific
version incompatibility problem using CSS. 
(See code comments.)

This has already been discussed here:
https://github.com/hypothesis/h/pull/1792#issuecomment-66702371

Various opportunities for simplification were proposed, but in the end,
the conclusion was that they won't stand up to scrutiny...
... except using `parseFloat()` instead of `parseInt()`,
which I have now done.

Any further comments?
> Can you make this last `0` result in a final `else` clause? It looks out of place, and like it could be a typo the way it is. Better to keep all the return values aligned, I think.

No. The whole point is that we only return *anything* inside the cycle when we found a difference. If there is no difference between the elements at that point, we keep going to the next element, without returning anything.

The final `0` is returned outside of the cycle, if we found no difference whatsoever. It does not belong inside the cycle.

However, I can add a `return` before it, if it makes it more comfortable to read it, in your opinion.