Added discount mechanism

Stored procedures `checkin` and `checkout` both modify the number of
discount tickets per station
Added procedure for taking discounts

GetDiscount takes a rider_id and a station and adds the rider to the
discounts table and subtracts the available discount. If no discount is
available, the function throws a runtime exception.
Added Client Callback Constants

Made the Client use the callback constants
Renamed GetDiscount to AcceptDiscount

To match the slides.
Fixed project builder

Fixed the declaration og acceptDiscount in the project builder and
removed unnecessary imports.
Update Client

Updated client to explain in more detail which procedures were being
fired off.