Add endpoint for bidding up auction

[pietre]

• add /bid-up endpoint to auction resource

• set logged user as winner

• increment price by 1 gr

• increment end date by 15 seconds

• decrement bids from user account

Error handling:

• throw exception if current datetime exceeds endDate
• throw exception if user doesn't have enough bids
• don't map exceptions to correct http status codes, just leave it as 500 for now (exception handler to be added later on)

[Rach-Coster]

Hey, I'm more than happy to do this one. Is there do you have a preferred structure for the PR?