Removed ResourceDiscoveringInterface

At the moment, there is no clear use case for the interface, so it's better
to remove it and introduce it again when we have a better idea about its
requirements and use.
Changed ResourceRepository::add() to retrieve source paths from another locator
Renamed ResourceInterface::get*Path() methods to better convey their meaning (closes #7)
Returned ResourceCollection objects everywhere
Updated CHANGELOG
Added information about resource collections to README