added new share type with cutoff date and incremented protocol version

added new share type with cutoff date and incremented protocol version

Changes:
    * shares contain a "desired_version" field that votes to warn other users about upgrades
    * shares contain a "far_share_hash" field that references its 100th parent
        * could be used in the future to implement a query/response check that can verify that all shares in a chain are present and that a competing chain actually has a lot of hash power behind it,
          preventing attacks where p2pool is flooded with shares that it is forced to remember for minutes
    * shares contain a merkle branch to the reference hash at the end of the generation transaction that will let other MM chains use p2pool's MM method
    * the "script" txout script bug is fixed

The switch time matches BIP16's (April 1st) for the Bitcoin chain and is March 27th for the Litecoin chain.