Using randombytes package instead of crypto

[niklabh]

using randombytes package instead of crypto as it helps in elimination of extra crypto libs inclusion if the code being used for the browser.

webpack include crypto code e.g. sha1 elliptic bn in the bundle if the crypto library is being used.
With this commit there is about 600K size reduction in bundle size for webpack.

with crypto:

with randombytes:

[CezaryDanielNowak]

Fixes issue #27

[dzek69]

I'd like to see a benchmark for node here. Using JS code instead of built-in compiled c++ module is probably (much?) slower.

Sticking to native solutions is almost always better choice. You can use an alias in webpack in the meantime for browsers.