Allow selection of extending documents

To allow:

````php
/** @Document */
class Foo
{
}

/** @Document */
class ExtendsFoo
{
}

$qb->from('Foo')->getQuery()->execute(); // will return Foo's and ExtendsFoo's
````

- Adds new multivalue node type property phpcr:classparents
- Popualtes this with the class names of ALL extending classes on
  persist
- QueryBuilder selects on phpcr:class OR phpcr:classparents

This is just a quick WDYT, so no tests yet. But it works

If this is good:

 * perhaps it would be a good idea to add a command to "sync" the registered nodes with their documents and their extending classes?, e.g. if user decides to make an already persisted document type a child class
 * due to a bug in doctrine-dbal this won't work with multiple parent classes (can only select on one value with that transport)