provide namespace alias for documents

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | |no
| Tests pass?   | travis
| Fixed tickets | -
| License       | MIT
| Doc PR        | Should we update the doc to use namespaces where possible? I guess only when symfony 2.6 is out and the sandbox running on 2.6

This will only do something with symfony 2.6+ and phpcr-bundle 1.2+, but is just passing an ignored additional parameter on older versions. Still lets wait for https://github.com/doctrine/DoctrinePHPCRBundle/pull/158 before merging.