Implement Naive BiddingAgents for testing

[shangwyoung]

Some naive strategies are:

• RANDOM: For each object, bid uniformly at random between 0 and <budgetRemaining>
• PROPORTIONAL: Sum up the numbers on all cards (<total>), and on the current card (<current>). Bid <budgetRemaining>*(<current>/<total>)
• COPYCAT: Bid the price from the previous round (or <budgetRemaining>, if that's smaller).