Difficulty - integer of the difficulty for this block. 仅仅此区块的难度。
Total Difficulty - integer of the total difficulty of the chain until this block. 从区块链的第一个区块到此区块的难度的总和。
例如：
{
difficulty: 759585,
extraData: "0xd783010409844765746887676f312e362e32856c696e7578",
gasLimit: 4712388, gasUsed: 0,
… … …
… … …
timestamp: 1467943085,
totalDifficulty: 10000790891,
transactions: [],
transactionsRoot: "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421",
uncles: []
}
