Always use strong source of entropy if available.

- Use window.crypto.getRandomValues directly in prng when
  reseeding if there is not enough data in first pool.
- Do not collect potentially weaker sources of entropy if
  window.crypto.getRandomValues is available, though Fortuna
  should offer some protection against an attack here?
- Default (random.js) plugin relies solely on
  window.crypto.getRandomValues if available now; this is
  presumably the behavior apps using JS will take.
forge.random.seed(): contribute entropy to the singleton RNG

Web workers don't have access to window.crypto.getRandomValues, so this
allows callers to pass "strong" entropy into the forge.random generator.