Add support for PDF url and fingerprint detection

1.

Now the PDF plugin is able to properly detect the
URL and fingerprint of the PDF document being viewed
by PDF.js.

However, this support is not leveraged by Annotator core,
for now. (It's up the the application consuming Annotator
to activate it.)

2.

Add minimal metadata extraction ability to the PDF plugin

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.)
Add more comments
Simplify url extraction
PDF: Handle documents with null title
Fix PDF URL extraction for FF, again
Merge remote-tracking branch 'hypothesis/pdf-update' into typed-packaging