Use go mod, add desc param to ERC20Transfers()

Merge pull request #19 from nanmu42/develop

add desc param to ERC20Transfers()

Workaround for missing tokenDecimal for tokentx calls

Some ERC20 contract does not have valid decimals information in Etherscan.
When that happens, TokenName, TokenSymbol are empty strings,
and TokenDecimal will be 0, instead of an unmarshaling error.

More information can be found at: #8