save number of unique user in each city
don't show progress while building feature vectors
Use twitter id to identify visitors

Before, I use 'uid' which means twitter id in old checkin and Foursquare user
id in new ones, therefore potentially causing same user to appear twice.
add sphinx documentation template
plot correlation between all venues features
use external function to compute entropy
move compute_entropy to avoid circular dependency
compute entropy of venue with respect to time of the day