Update TODO.txt
Added a test to UserBalance for multiple balances
Moved helper methods out of UserBalance for other models to use
Working on user.depts method
Added previous_user_balance_id column to UserBalance

This will allow to track balance history and make it easier to pull
information like depts and credits by finding the last balance with
previous_user_balance_id set to 0
Made methods return objects
Working on depts method for User
Added tests for previous_user_balance_id in UserBalance
UserBalance -> added column current, fixed update_balances

- new column to track latest balance -> current
- update_balances
    - set process_flag on UserDept and UserPayment on successful process
    - set current to false for old balances