[CRX] Get pdf name from URL instead of query string

Fixes https://github.com/operasoftware/pdf.js/issues/6.

Before this fix, http://www.berlin.de/imperia/md/content/lb-patienten/patientenforum/martus_gute_studie.pdf?start&ts=1309357143&file=martus_gute_studie.pdf was incorrectly treated as http://www.berlin.de/imperia/md/content/lb-patienten/patientenforum/martus_gute_studie.pdf, causing the viewer to break in the Chrome/Opera extension.