Split cache

Memcache on heroku does not support value > 1M. 

Which make [nbviewer2 to throw 500](http://nbviewer2.herokuapp.com/urls/raw.github.com/jiffyclub/ipythonblocks/master/demos/starry_night_to_text.ipynb)

This [fixes it](http://gistpynb.herokuapp.com/urls/raw.github.com/jiffyclub/ipythonblocks/master/demos/starry_night_to_text.ipynb) but at a performance cost because of pickle and speading across multiple keys. 

I'll try to improve it by skipping this logics for value <1M.