Add private name sales to consensus

[QuickMythril]

This branch adds the functionality for private name sales, allowing someone to specify which address can buy, when selling a name. This would also allow them to set the cost to 0 (for private sales only), effectively "giving" someone a name, though it would still require the recipient to "accept" (purchase for 0). This implementation is complete, however it should be tested network-wide on a testnet. Also before mainnet release, it should probably set to trigger at a specific block in the future, to ensure that all nodes will be updated before any changes to consensus. This code has been reviewed and planned very carefully and deliberately, and even some of the variable names were chosen specifically to avoid issues or further complicating things. However, if there are any questions or suggestions for alternatives, any responses here are greatly encouraged.