PDF support

This is the other half of the PDF work (together with https://github.com/hypothesis/annotator/pull/11)
Also includes the separation of the PDF work and fuzzy macthing into a plugin.

H only received minimal changes; the crux of the matter happens inside Annotator.

This obsoletes https://github.com/hypothesis/h/pull/866 and https://github.com/hypothesis/h/pull/870.
(I have just rebased this to latest develop, after the heatmap arrow scrolling fix.)

Demo server is at http://pdf10.dokku.hypothes.is/