Zerobinpaste updates: seed prng from /dev/(u)random, shorter keys

Finally got around to poke js for alternative encryption methods and haven't really found anything marginally better to justify the effort of swapping the current scheme - shorter keys can already be fairly safe due to pbkdf2 in place (though 1k iterations isn't much at all).

What I found however is that sjcl doesn't seem to be written with node.js in mind and its `sjcl.random` doesn't try to seed prng (used for key and ccm nonce) from system entropy sources like `/dev/random` and `/dev/urandom`, so 415516e addresses that.

118b0ce uses much shorter 48-bit keys (8 chars), still allowing user to configure arbitrarily-longer ones (should be easy to do via `~/.zerobinpasterc`).
That isn't much, but still probably more than most of the user passwords/passphrases have.
Given that superb security against such attacks isn't vital here (decryption js is quite likely to be served over http/bad-tls anyway), I think it's a sane default with a length trade-off in mind.
If not a good default, can be changed to anything in the option parsing code there, but the option itself is still worth having, I think.

Combined with cb8e52b (will open separate PR), urls go from this:

    http://some.0bin.site/paste/9611846d1c86e549fabe205837d40b780f5efce3#bLilFEQok1qTOzifX2gdvZO1ZoHki1getSt/YnAdAFQ=

To this:

    http://some.0bin.site/paste/FxsUsOGB#aW8DhwMy

As these keys are purely client-side, there should be no compatibility issues of any kind.