Enforce prefunding based on the auction type

[0xJem]

• Implements an auctionType() function with Atomic and Batch as the enum values
• Enforces that a batch auction must be prefunded, while an atomic auction is not
• Removes a lot of the implementation and test code that was present to handle prefunding state

[0xJem]

The implementation looks good. I'm a bit torn on not allowing prefunding for Atomic Auctions. Technically, you could prefund them and it would reduce gas costs on purchases a bit + allow sellers to deposit tokens instead of having them withdrawn from their wallet. However, having batch always require prefunding and it be optional for atomic seems like it could add some complexity.

The prefundingRequired return value from the auction module offers a way to override the default. I can add it back?

[0xJem]

allow sellers to deposit tokens instead of having them withdrawn from their wallet

It's more withdrawing tokens in one go vs every time there's a purchase, no?

[0xJem]

Superseded by #73