improved happy_hour? method

—gives more options to have HH at times not on the hour in the future —
i.e., 3:30-5:30pm
Bar.purchase method

with helper variable @number_of_purchases in MenuItems and
MenItem.increment_purchase method
#analyze_popular_drinks

close, but sort_by not sorting properly
added Purchase class

refactored purchase, MenuItem
created Purchase class
#analyze_popular_drinks working
moved many variables into a before do section
tests written for special discounts
set_special_discount working
refactored to put special_discount in MenuItem
remove_special_discount
wrote tests for analyzing popular drinks
sorted array of popularity during hh