Adapts partially to Py3. Adds a known good npm/nodejs download channel.

[loriab]

Far from comprehensive, but catches a few Py3 problems I saw in trying to run Tutorial 3. Also specifies conda-forge nodejs package as a worthy way to get "npm", rather than the broken npm conda package on cpcloud channel.

I notice afterward your py3 branch, so let me know if you'd rather have this PR to that.

Current error for Tutorial 3 is:

... last 1 frames repeated, from the frame below ...

~/linux/notebook-molecular-visualization/nbmolviz/widgets/selector.py in __getattr__(self, item)
    105 
    106     def __getattr__(self, item):
--> 107         if self.viewer is not None: return getattr(self.viewer, item)
    108         else: raise AttributeError(item)
    109 

RecursionError: maximum recursion depth exceeded while calling a Python object