Add static return types in phpDocs

Allow for proper chaining of functions
`MyModel::where('..')->orderBy('x')->first()` should return instance of
`MyModel` again.
Doesn't work in every IDE, but is described in https://github.com/phpDocumentor/phpDocumentor2/issues/800
It does work in phpStorm/NetBeans, and doesn't seem to have side-effects in other IDEs like Sublime