Add static image link to `https://qtum.info/api/qrc20`

would be nice to have a new field introduced `image_url: https://qtum.info/dist/img/27ca638.jpg`

To facilitate the integration of QRC20 in external application:

- Getting Information about the token by the contract
- Download the image of the tokens through image_url
- Restart your application
- Enjoy QRC20 all set just with the contract