Update bitcoinrpc.cpp

listsinceblock method added - allows to get all transactions in blocks since block [blockhash], or all transactions if omitted. This method is very important for monitoring incoming transactions mechanism.
Merge pull request #44 from olgasanko/patch-1

Add RPC call "listsinceblock"
listsinceblock method returns transactions that belong to our wallet since the given block.
Response contains current 'last block' and an array of correspondent transaction objects.