Added flag prohibiting GraphDiff from ever removing collection elements ...

...from the database (owned or associated). An attempt to provide a config for issue #105. Currently this still loads (has to) **all** elements of the collection from the database (to be able to compare, what's new/updated etc., while attaching). So this implementation quite definitely has performance issues for the case it is intended to support, because the number of elements in the collection would only ever increase. Probably an (unchecked) append-only mode for collections would be of more value to support this case.

@refactorthis: Do you think this is a useful feature, so I'd be worth investing some time exploring an append-only mode? Would you prefer to do this via the fluent API and/or attribute mappings instead of a global configuration to be more flexible / useful?