Categorized ERC721 token

[joaoaguiam]

This PR propose a new feature to the OpenZeppelin project by proposing an extension of the ERC721 Token in order to be able to categorize the tokens managed by the ERC721 token.

🚀 Description

We propose this extension of the ERC721 in order to allow the categorization of the tokens managed by the ERC 721 standard. We can therefore support both Fungible and Non-Fungible tokens with a single token.

This new extension was developed in the scope of the ETHDenver Hackathon by the Taiga Market team which is building a decentralized marketplace for fungible tokens, such has game assets or digital assets where all tokens belonging to a category have the same properties.

• 📘 I've reviewed the OpenZeppelin Contributor Guidelines
• ✅ I've added tests where applicable to test my new functionality.
• 📖 I've made sure that my contracts are well-documented.
• 🎨 I've run the JS/Solidity linters and fixed any issues (npm run lint:all:fix).

We will propose the need tests/changes in the following days.

[spalladino]

Thanks @joaoaguiam! We'll be revisiting this PR as soon as we finish the implementation of the latest ERC721 standard in #803.

[nventuro]

Closing due to staleness.