[WIP] Feature/search delete from index

Search deindexing on node delete and delete event from content mapper

- Added `loadShallowStructureForNode` method to ContentMapper
- Send out event for each deleted node and provide a Shallow structure to the listening events (in addition
  to the PHPCR node being deleted)

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | [yes|no => <WHY?>]
| Fixed tickets | [<WHICH?>|none]
| BC Breaks     | NO
| Doc           | NONE

I use the PHP 5.5 yield keyword here, which means you can do:

````
foreach ($event->getStructures() as $structure) {
    break;
}

$structure // first structure
````

and only the first Structure will be "yielded" instead of creating an array of structures and returning them all.

This functionality is the equivilent of creating an Iterator.