sha256 hash for pdf files using digest.js

pdf.js will use digest.js for sha256 hashes. I added sha256 function alongside fingerprint function so we can keep both, without changing core's functionality.
The code is taken from here: https://github.com/peerlibrary/peerlibrary/blob/master/client/crypto.coffee
but translated into JavaScript. Hash is computed in chunks. We should agree on the chunk size.
I added comments for every changes on pdf.js core so it's easily tracked in the future.
I extended the list of worker_loader files because digest.js should be loaded in web worker.