Batch should define Hash method

[tzdybal]

It is crucial that both the client and server side of go-sequencing API is using exactly the same method of hashing transactions from a Batch. To ensure this, Batch.Hash() method should be defined. Without this, lastBatchHash parameter is very fragile to any implementation changes..

In case of Go implementation, this method will be available for programmers. It will also serve as an example for implementations in other languages.