Extract REST API in a default MP Package

As a developper i should be able to extend the API, in order to expose my own enpoints
Acceptance:

• The directories REST api impl is available in "sample" addon (commited on a side project, not in the main build)
• Extension point is correctly documented
• The following documentation page (currently visible to Nuxeo group only) http://doc.nuxeo.com/display/NXDOC/Contributing+a+new+endpoint is completed with the steps to follow and the main classes/objects that should be used