Teach prun.rb to report what it's actually pruning..
Teach control and stop do do nothing with nil node:

* This mimics the behaviour of the fn play

control nil, foo: 1, bar: 2 # does nothing, returns nil

stop nil # does nothing, returns nil
We're in v2.3.0 dev territory
New fns: with_bpm_mul, use_bpm_mul

* Allows users to change the current BPM as a
  multiplication of the current BPM

* use_bpm_mul 2 
  will set the BPM to be twice its current value.
Give each run a new rand generator solely for new threads:

Previously the standard thread local random generator
was used to create a new generator for each thread
in a deterministic manner. However, if multiple threads
were created sequentially the previous approach gave
each thread a similar generator (i.e. with the same seed).

This new approach uses a separate generator for generating
new thread generator seeds. This allows the standard 
generator to not be polluted with new generator generations
in addition to giving each new thread's generator
a new seed in a deterministic manner.