Make pdf.js use absolute file urls, and get PDF URl from a JS variable

Bug 1316407: By default, this version of pdf.js wants to take the PDF
URL from a URL query parameter. It also wants to refer to several of its
JS libraries by relative paths, which sometimes conk out in Mahara.

Change-Id: I1be894beb825bae4070e6e43c5900d43313ef583