feature: support for delegated ERC20 (OpenZeppelin version)

[lucasmenendez]

We need to support a flavour of an ERC20 token created by OpenZeppelin that allows to delegate voting power to other holders, and return the correct ammount of voting power exposing a special method for that: getVotes(sender).

This task includes the compilation of the OpenZeppelin ERC20 to Go and the creation of a new TokenType that identifies this new token. The holder scanner will use the getVotes(sender) method to track the balances of holders of this token.