previewer: initial pdf.js integration

* Integrates pdf.js support in Invenio including customization of
  default pdf.js viewer.

* Fixes problem with pdftk bundle naming collision.

* Fixes problem with jQuery loading in records module.

Reviewed-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Signed-off-by: Adrian Pawel Baran <adrian.pawel.baran@cern.ch>
previewer: zip preview and styling fixes

* Fixes issue with ZIP files with no content.

* Fixes minor styling issues.

* Changes icons from Bootstrap to Font Awesome.

Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>