Implement `SharedRoyaltyTokenEnumerable` extention

## Overview

As a developer implementing an enumerable shared royalty token, I want a contract that implements the enumerable interface so that I can be assured I'm implementing the token to spec and don't need to write common code.

### Questions
- Is everything that we want to enumerate enumerate-able?

### Reference

- [Solidity Inheritance](https://solidity.readthedocs.io/en/v0.4.24/contracts.html#inheritance)
- Contract Archetecture #78
- [Etherium NatSpec](https://github.com/ethereum/wiki/wiki/Ethereum-Natural-Specification-Format)
- [OpenZeppelin 721](https://github.com/OpenZeppelin/openzeppelin-solidity/tree/master/contracts/token/ERC721)

### Assumptions

- Delivered in `bootleg-tokens`
- Can reference OpenZeppelin's 721 implementation

### Acceptance

- SharedRoyaltyTokenEnumerable interface defined in #78 documented with NatSpec
- Tests!

Start in the bootleg-tokens area and read Contract Architecture #78 to understand the SharedRoyaltyTokenEnumerable interface. Compare the intended interface with OpenZeppelin's ERC721 implementation and review the Ethereum NatSpec reference. Define the documented interface and add tests covering the enumerable behavior described by the project.

Written by the indexing model from the issue text.