WIP previewer: pdfjs plugin addition

* Adds pdfjs previewer.

Signed-off-by: Adrian Pawel Baran <adrian.pawel.baran@cern.ch>
The official bower package for `pdfjs` is `pdfjs-dist`, but this one can be probably only used to build own version of the viewer and it doesn't contain the official version of the viewer (the one I am using here).
The two example ways of how to obtain this official viewer are:

1) Use the official repo https://github.com/mozilla/pdf.js as endpoint for `bower install` and then build the viewer locally (`node make generic`)
2) Use the 3rd party repo (eg. git://github.com/RiptideCloud/pdfjs-build.git), which comes with the prebuild version of the viewer

And then, there still remains the problem with some paths hardcoded in the scripts (like `PDFJS.imageResourcesPath` for example), which we need to customize in order to make the viewer work in invenio. 

The simplest solution which comes to my mind (as I mentioned in the first comment) is to fork the 3rd party repo and make the changes we need.

But I'm not sure if this is the best way to solve this.