The main purpose of the change was to port Robohash to Python3.
I took the opportunity to go back and fix a few things that I should have done better the first time.

+ Port to work with both Python2 and Python3
+ Port to run with newest Tornado-
+ Dynamically read in list of sets from filedir (this won't be slow, since we run out of a ramdisk in prod)
+ Break Robohash object out into it's own class, so we can call it from other programs without going through HTTP
+ Allow /set_set3/ style parameters anywhere in the URL
+ DataURI as PNG instead of jpg - I'll make this configurable in a future version.