Changed User class so that user_id is not an optional parameter on init.

 - Changed flask server to init users with user_id as first parameter
 - Modified __init__ in User class: changed USER_ID to have a value of None, and to run timeline request only if self.USER_ID is truthy
Added if/else statements to Score class score_by_hashtags to handle case of 0 hashtags used.

Division by 0 caused an error.
Added if/else to return 0 if num hashtags is 0.
Move check_rate_status function and associated global vars into flask app
Main() instantiates a test user with my username in friends.py