Default random generator seed in all new threads to 0

* This makes the random streams to be much less tightly coupled allowing 
  you to move threads around in a composition without affecting the 
  stochastic aspects of the sound due to random generator differences.
* Give in_thread a :seed option to allow the explicit specification of
  a seed for the new thread:

  in_thread seed: 2 do
    puts rand #=> 0.436 
  end