Set attributes of picker object directly for the primary picked object

This makes it easy to link attributes (like, for example, the point picked) with attributes of other objects.  For example, this example tracks the mouse movement across the sphere with a small red point:

username='jasongrout'
html("<script src='http://boxen.math.washington.edu/home/jason/pythreejs/pythreejs.js'></script>")
load('http://boxen.math.washington.edu/home/jason/pythreejs/pythreejs.py')
html("<script src='https://raw2.github.com/%s/ipywidgets/master/widgets.js'></script>"%username)
load('https://rawgithub.com/%s/ipywidgets/master/widgets.py'%username)

import time
print 'Loading...'
time.sleep(2)
my=[]
my.append(Mesh(geometry=SphereGeometry(radius=4), 
                material=LambertMaterial(color='white')))
red = Mesh(geometry=SphereGeometry(radius=.1), 
                material=LambertMaterial(color='red'))
my.append(red)

c = PerspectiveCamera(position=[0,10,10], fov=40, children=[DirectionalLight(color=0xffffff, position=[3,5,1], intensity=0.5)])
scene = Scene(children=my+[AmbientLight(color=0x777777)])
p=Picker(event='mousemove')
renderer = Renderer(camera=c, scene = scene, controls=[OrbitControls(controlling=c),p])
show(renderer)

a=Link(widgets=[(p,'point'), (red, 'position')])
show(a)