Renamed DomainObjectCollection to Collection
DomainObject Collections do not need the DomainMapper

The reason I injected the mapper to the domain object collections was with the idea
of persisting collections in mind. I thought I could retrieve from them the mapper
which knows how to deal with the objects the collection contains, but it doesn't feel
like a good approach. I think I can get something much cleaner once I introduce a
Unit Of Work.

I'm starting to think I probably don't need $modifiedProperties and $modifiedAggregates
in the DomainObject either. We'll see later on.