Introduce EntityRepositoryInterface, ancestor for custom repositories

[Majkl578]

Introducing a common ancestor for custom repositories.
Currently there is a slight mess, some pieces of code only specify ObjectRepository which:
a) is insufficient since it's missing Selectable;
b) is incorrect since following code expects EntityRepository.

I've taken the opportunity to also add return types to the interface and default repository. Also updated some phpDocs - specialized array and removed duplicate information.

This change should not affect 99.9% of user code since almost everyone extends EntityRepository, but allows 1% to use custom repositories in fancier way.

TODO:

• constructor should be part of the interface or made parameterless
• make Scrutinizer happy

[guilhermeblanco]

This interface should be part of Doctrine\Persistence module, and also we do not suffix with Interface

[Majkl578]

We already have ObjectRepository as a base interface. This interface extends it with ORM-specific methods like createQueryBuilder or by adding Criteria support. I don't think those are methods that'd belong to base persistence layer, i.e. ODM doesn't support Criteria.
Suffix is there because EntityRepository is already taken by default imlementation.