Adapt to support PDF.js v1.0.907

... as per #86.

Tested with:
 * Iceweasel 31.2.0 (Shipped with Debian, built on top of Firefox ESR) - ships with PDF.js v1.0.68
 * Firefox 31.3.0 (current ESR a.k.a. Extended Support Release) - ships with PDF.js v1.0.68
 * Firefox 34.0.5 (current stable) - ships with PDF.js v1.0.712
 * Firefox 34.0.5 (current beta - yes, it's the same as stable, just released) - ships with PDF.js v1.0.712
 * Firefox 35.0a2 (developer edition a.k.a. aurora a.k.a. alpha) - ships with PDF.js v1.0.907

According to tests, all involved PDF.js versions are now fully supported.

> Here's your problem: https://github.com/hypothesis/annotator/blob/support-pdfjs-1.0.907/src/plugin/pdf.coffee#L263
> There you're calling it without the scope, which is why you need a fat arrow. Stop doing that.

Done in ad79593f202b360a2bf58cad7dd0812ececc3334.

> I don't know why you're calling `@annotator.domMapper._waitForInit` there. Isn't `@annotator.domMapper' the same as `this`? Isn't it the `PDFTextMapper` instance where this is being called?

No. This is the `Annotator.Plugin.PDF` instance. It's slightly confusing to have two classes in the same file; they have already been separated on other branches, but not no the current (legacy) one.