Release 2.1.5

Merge pull request #35 from sullof/minor-fix

Fix minor bug

Merge pull request #32 from TronWatch/optionalPrivateKey

Make signing optional for sendTransaction and sendToken

Update README.md

Merge pull request #19 from tronprotocol/2.0.4-apis

2.0.4 apis

updating node example

change judge condition of deploy contract

add log if call_value is wrong when contract is deployed