Allow empty merkle trees.
Allow empty merkle trees.

An empty merkle tree is defined to have a merkle hash of zeros. This
patch implements that behavior in the calcMerkleRoot function.

bankonme/bitcoinjs-server,javascriptit/bitcoinjs-server,dazoe/bitcoinjs-server,zhangweis/node-jds,dazoe/bitcoinjs-server,dazoe/bitcoinjs-server,bitfloor/bitcoinjs-server,bankonme/bitcoinjs-server,zhangweis/node-jds,javascriptit/bitcoinjs-server,bitfloor/bitcoinjs-server,zhangweis/node-jds,javascriptit/bitcoinjs-server,bankonme/bitcoinjs-server