ERC-777 A New Advanced Token Standard (#907)

* ERC777 A New Advanced Token Standard

* ERC777: Add ERC777 Logo

* ERC777: Normalize EIP/ERC names

* ERC777: Spec for tokensToSend when burning tokens

* ERC777: Update official repository

* ERC777: Clarification and corrections

- Clarify unclear sections of the spec
- Fix typos and grammar mistakes
- Improve aesthetics of the text

* ERC777: Change terminology of the repo, small fix

- Don't refer to the repo as the official repo but the repo of the
  reference implementation
- Fix a small typo in the AuthorizedOperator event

* ERC777: logo & release to public domain (CC0)

* ERC777: Use markdown not html & relative links

* ERC777: Adapt to new EIP template

* ERC777: Use solidity syntax and fix relative links

* ERC777: Add discussions-to link

* ERC777: Fix link in discussion-to

* ERC777: Fix image links

* ERC777: Fix eip type

* ERC777: Update header

Author: jbaylina