PDF: update for latest PDF.js + metadata support

This PR brings several changes for the PDF support.

 * Now the PDF plugin is able to properly detect the URL and fingerprint of the PDF document being rendered by PDF.js, even if the URL is distorted by the chrome plugin.
 * Metadata extraction: From now on, the Document plugin is not supposed to be loaded, when using the PDF plugin. The PDF plugin is now replacing this functionality for PDF. (Currently, it can only extract URLs and title, but that's all we have or need for now.)

Solves #72.

  * * *

Don't merge this before https://github.com/hypothesis/h/issues/1407 is fixed, because as of now, this code has a temporary workaround, which should be removed.
Not having to write `16` would be clearer, but on the other hand, I find the code to be more human-readable this way.

(Obviously, it would be best to have a `startsWith()` method on the String type...)

But I have don't have a strong preference. If any of you does, we can change it.