Move registry plugin to core

[ricardograca]

• Related Issues: Promote some useful plugins to core #1806

Introduction

Move the registry plugin to core Bookshelf.

Motivation

This is a useful plugin and is the best way to ensure there are no circular dependencies issues when models are located in different files.

Fixes #1557.

Proposed solution

Functionality is moved to the bookshelf.js file. Note that registered models and collections are now stored in the bookshelf.registry.models and bookshelf.registry.collections respectively. This means the change is not backwards compatible but it's not expected that users were accessing bookshelf._models and bookshelf._collections directly anyway.

A lot of the code was modernized.