Ensure with/use_bpm use floats for division
New fn: density

summary: "Squash and repeat time",
doc:     "Runs the block d times with the bpm for 
          the block also multiplied by d. Great for 
          repeating sections a number of times 
          faster yet keeping within a fixed time."

example:

live_loop :foo do
  density 1 do
    sample :bd_haus
    sleep 0.5
  end
end

# Now change density to 2, then 4, then back to 1
Teach Float#times to yield a float to the block:

3.times do |el|
  puts el
end
# Spits out 0, 1, 2


3.0.times do |el|
  puts el
end
# Spits out 0.0, 1.0, 2.0