Implemented counting of usedGas

ethchain/state_manager.go

@@ -140,6 +140,9 @@ func (sm *StateManager) ApplyTransactions(state *State, block *Block, txs []*Tra
 		validTxs = append(validTxs, tx)
 	}
 
+	// Update the total gas used for the block (to be mined)
+	block.GasUsed = totalUsedGas
+
 	return receipts, validTxs
 }