External figs

@BRONSOLO @mkcor @pedrodz @chriddyp 

Our NBs (mostly mine though) display several images hosted on other sites (e.g. imgur). 
With this PR we'll be able to host these images in streambed/ effortlessly. 

- adds  `image_map.py` script
- allows to automatically download images with https/http sources
- allows to automatically change the image sources in the published html files to their streambed location
  
This PR also features some re-organization of the NB to plot.ly process (but just re-organization)