solidity之常用库网址：
https://github.com/pipermerriam/ethereum-datetime
https://github.com/ethereum/dapp-bin/blob/master/library/iterable_mapping.sol
https://github.com/Arachnid/solidity-stringutils
https://github.com/OpenZeppelin/openzeppelin-solidity
https://github.com/OpenZeppelin/openzeppelin-solidity
https://github.com/ethereum/dapp-bin/tree/master/library
