In order to make it easier to change the document metadata retrieved for
PDFs, this commit moves the code responsible for extracting that
metadata into a separate (tested) service.

We also remove our reliance on the `PDFViewerApplication.loading` which
was removed from the bundled version of PDF.js in August 2015. Instead
we simply check to see if `PDFViewerApplication.documentFingerprint` is
available. If not, we listen for the first 'documentload' event before
extracting metadata from the `PDFViewerApplication`.