Replaced Database Connection instances and added the ability to easily swap models

[kevindierkx]

Replaced Database Connection instances in favor of repositories, comparable to: #90

Added support to swap models, possible solution for: #58

// On repositories
AuthCodeRepository::setModel(SomeOtherModel::class);
ClientRepository::setModel(SomeOtherModel::class);
RefreshTokenRepository::setModel(SomeOtherModel::class);
TokenRepository::setModel(SomeOtherModel::class);

// On models
Client::setTokenModel(SomeOtherModel::class);

[taylorotwell]

Tests are failing. Please re-submit when tests are passing.