Add an interface and a generic implementation of the [ERC721](https://github.com/ethereum/eips/issues/721) token standard. Review: - @eordano's [implementation](https://github.com/ethereum/eips/issues/721#issuecomment-347046746) for [Decentraland LAND](https://github.com/decentraland/land/blob/master/contracts/BasicNFT.sol) - Cryptokitties team [implementation](https://github.com/axiomzen/cryptokitties-bounty/blob/master/contracts/ERC721Draft.sol) of Kitties - Dharma's generic token [repository](https://github.com/dharmaprotocol/NonFungibleToken)
Add an interface and a generic implementation of the ERC721 token standard.
Review: