Feature/openzeppelin erc20 support

[lucasmenendez]

Solving #92:

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 amount 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.

There are two possible log events to be tracked:

• LOG_TOPIC_ERC20_TRANSFER (0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef): Which is emitted when a holder transfers some balance of the token to other address.
• LOG_TOPIC_ERC20VOTES_DELEGATE (0xdec2bacdd2f05b59de34da9b523dff8be42e5e38e818c82fdb0bae774387a724): Which is emitted when a user voting power changes. So when a user delegates some power voting, this event will be emitted twice: the first time includes the address, the previous and the current power voting of the user who performs the delegation, and the second time includes the address, the previous and the current power voting of the user who receives the delegation.