Implement `SharedRoyaltyTokenEnumerable` extention

[barlock]

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
• Contract Archetecture Contract (token & application) Architecture diagrams #78
• Etherium NatSpec
• OpenZeppelin 721

Assumptions

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

Acceptance

• SharedRoyaltyTokenEnumerable interface defined in Contract (token & application) Architecture diagrams #78 documented with NatSpec
• Tests!