add Python API for adding jupyter js widgets to easyform

[scottdraves]

this should work, like the similar code in groovy:

from beakerx import *
from ipywidgets import * 
w = IntSlider()

f = EasyForm("Form and Run")
f.addTextField("first")
f.addTextField("last")
f.addWidget("slider", w)
f['first'] = "First"
f['last'] = "Last"
f.addButton("Go!", tag="run")
f