[WIP] Support multiple @find-by annotations

Still open tasks but basic support for multiple ```@find-by```. Still requiring some new unit tests and ```ElementCollection``` see #48 for more details.

Closes #13
1. collection proxy has only 1 element inside it and it's a collection and we're forwarding all calls to it
2. the `setContainer` method on a collection will propagate container to each collection element

Knowing that it's possible to access collection object via `$this->getObject()` inside collection proxy, is it not? If that is so, then do a fix here and in another similar place.