Inheritance on a Stream seems to be broken.

The following example fails with 'map' object has no attribute 'draw' :

import streamz

class MyStream(streamz.Stream) :
    def draw(self) :
        print('called draw')

@MyStream.register_api()
class draw(MyStream) :
    def __init__(self, *args, **kwargs) :
        print('called draw')

s = MyStream()
s.map(lambda x : x + 5).draw()
s.emit(99)

I couldn't find a class factory in the source and it looks like all the nodes are instantiated directlly with Stream when it should use the type of the main instance.