Now collects random samples of comments & tracks

all_data_one_person.py has now become all_data_one_entity.py, and can
collect the data we want about one user, one track, or one comment.
rand_samp.py can now collect random samples of any of these three, with
use of an extra keyword argument for its collect() function. It also
has a function called seek_limit() which is useful in estimating the
lowest actually-used id number for tracks, users, or comments.