Faster chromium version

[fvsnippets]

Hi! this gist (another implementationt of scrypt also based on Web Crypto) seems to be REALLY noticeably faster when running in chromium derived versions (tried on Google Chrome and Iron Browser).

Also tried in Firefox but it doesn't work due to this bug (yet more info here) . Happily it can be easily fixed using this lib also by juanelas. Once applied the fix, the speed is much similar to scrypt-pbkdf but still slighty faster.

Haven't tried with other browsers and I do not know whether the implementation is correct or not. I am not the developer of the gist and frankly the math behind cryptographic algorithms I don't think is my thing so I can't say.
But I think it's worth it to @juanelas to take a look.

Hope it helps! (And thanks for your work!)