Don't call .now anymore

```ruby
class Button
  def push
    emit "anything"
  end
end

button = Button.new

stream = Frappuccino::Stream.new(button).map{ |event| 1 }.inject(0) { |sum, e| sum +=e }
button.push

s1 = stream

puts s1 # => 1
button.push
s2 = stream
puts s2 # => 2

push button
puts stream #=> 3
```