Update upstream source from tag '2.3.0'

.gitattributes

.gitignore

.scrutinizer.yml

.travis.coverage.sh

.travis.install.sh

.travis.yml

CONTRIBUTING.md

README.md

composer.json

couscous.yml

doc-template/css/main.css

doc-template/default.twig

docs/access-interceptor-scope-localizer.md

docs/access-interceptor-value-holder.md

docs/credits.md

docs/generator-strategies.md

docs/lazy-loading-ghost-object.md

docs/lazy-loading-value-holder.md

docs/null-object.md

docs/remote-object.md