lib,src: make pseudoRandomBytes alias randomBytes

Previously pseudoRandomBytes worked similarly to randomBytes but in the
event of insufficient entropy would silently return non-secure values.

As of f68a116c, the entropy pool blocks if there is insufficient entropy
instead of giving an error so there is now no longer a case where
pseudoRandomBytes would act differently than randomBytes.

Docs are updated to remove pseudoRandomBytes and to clarify that
randomBytes now does block instead of erring when entropy is low.

PR-URL: https://github.com/iojs/io.js/pull/557


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor@indutny.com>