prefer crypto:strong_rand_bytes/1

[getong]

I just see the cowlib use the strong_rand_bytes/1 to replace the rand_bytes/1.
So I add the code here, see
Fix #39

[Licenser]

merged and reverted again strong bytes can throw exceptions.

[Licenser]

Mate I appreciate your contributions and enthusiasm but I must ask you to be a bit more diligent before opening a PR. It's neither fun for you to have your changes reverted nor is it fun for me to nit pick over every change and investigate every function.

If you read the documentation of strong_random_bytes it will throw errors of the random source is out of data this can be quite fatal and doesn't sound like a very great idea.

[getong]

Sorry. I had read the doc before I made a PR, and I know to add the catch before the function, but I think it is worth doing it. A little change must be made.