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?
> I was not aware of the three dots, exclusive end syntax. I thought it was just the CS parser being forgiving.

No, it's a real feature :) Actually documented at http://coffeescript.org/#slices.

Although I agree that it's pretty crazy to design a language with different meaning for "`..`" and "`...`".
(I mean, what's next? Different meaning for spaces and tabs? Here we come, [Whitespace](http://en.wikipedia.org/wiki/Whitespace_%28programming_language%29)!) But then again, JS is crazy to begin with, so it does not hurt too much.