Add option to serialize merkle block with transactions

[braydonf]

This adds the ability to serialize the merkle block data structure to include the transactions, which would otherwise be omitted in the process. There are cases where it's necessary to serialize and persist a merkle block before the block is sent to the wallet.

[pinheadmz]

Changes look good to me, test passes and coverage is good.

Wondering about MerkleBlock.format() and .getJSON() with respect to the value txs. In a full block, both functions return a txs value that is a JSON array of each tx also JSONified.

In merkleblock, format()returns the number of txs, and getJSON() does not return txs at all. I think behavior of MerkleBlock should match that of Block, especially now that we are adding transactions to it.

[braydonf]

To include the transactions for getJSON, I think there would be a matching getExtendedJSON as the transactions are not included in default serialization, and there is a matching fromJSON(). I don't think that is really necessary at this point.

Added txs to format() to match that of block.