Expand flattened objects on insert

With this pull requests every `DBObject` to insert in its flattened representation, will be expanded to a its full BSON-structure.

_Example:_
The statement `db.collection.insert({"test.id" : "myId"})` will be persisted as `{ "test" : { "id" : "myId" } }`. This will result in a more mongo equivalent behaviour, since find/findAndModify-queries will return this expanded representation.

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@hoffrocket / @twillouer please review, merge and delete the branch.
you can use Util.split to have help with "Path"