Add 'chainwork' to getblock

Returns nChainWork from the block index, the total work done by all
blocks since the genesis block.

src/rpcblockchain.cpp

@@ -62,6 +62,7 @@ Object blockToJSON(const CBlock& block, const CBlockIndex* blockindex)
     result.push_back(Pair("nonce", (boost::uint64_t)block.nNonce));
     result.push_back(Pair("bits", HexBits(block.nBits)));
     result.push_back(Pair("difficulty", GetDifficulty(blockindex)));
+    result.push_back(Pair("chainwork", blockindex->nChainWork.GetHex()));
 
     if (blockindex->pprev)
         result.push_back(Pair("previousblockhash", blockindex->pprev->GetBlockHash().GetHex()));