Class D transaction type

Class D transactions include the reference public key and the public key of the Exodus address in a multisig output. A third public key in the multisig output may included embedded Master Protocol data.

The goal of the Class D transaction is to optimize Class B transactions by reducing size, cost, chance that multisig outputs will remain unspent, and in some cases, increase the amount of bytes available to embed MP data without increasing cost (over Class B transactions, that is).

Relevant: Find a clever trick to lower the cost of transactions, https://github.com/mastercoin-MSC/spec/issues/192