Make the unmarshallers pluggable on the Nuxeo object

Allow to register unmarshallers based on the 'entity-type' field.
Those unmarshallers will be used when retrieving a JSON response with an 'entity-type' field.

Add default ones for:

• document
• documents
• user
• group
• directory

Let the user be able to register custom ones:

Nuxeo.registerUnmarshaller('document', DocumentMarshaller);
Nuxeo.registerUnmarshaller('user', UserMarshaller);
...

Those unmarshallers will be called at the same place, in Nuxeo#_http method based on the resolved response.