Support nested associative arrays

It should be possible to have an array type which automatically creates a structure, e.g

``` php
class Site {
/**
* @PHPCR\AssocArray()
*/
protected $preferences;
}
```

Then automatically map that to a PHPCR structure:

```
/site
preferences/
- jcr:primaryType = phpcr:associative_array
- key1 = value
key2/
- key3: = value
- key4: = value
key5/
```

The issue names no implementation files or tests. Start by tracing how the Site::$preferences example would be mapped to the shown PHPCR structure, and consider the work done when nested associative arrays are persisted with the requested structure.

Written by the indexing model from the issue text.