text: Title: Namespaces should be lazy registerable
  Body: As @frol suggested, for big APIs splitted on multiple files, Namespace should be the only class to import.
  As Blueprints for Flask, it should provide all necessary methods to not require the API instance in the file and to be lazy registerable.
  
  The target is being able to write:
  
  ``` python
  # feature.py
  from flask_restplus import Namespace, Resource
  
  ns = Namespace('feature', 'Some namespace')
  
  model = ns.model('Model', {})
  
  @ns.route('/')
  class MyFeature(Resource):
      @ns.marshal_with(model)
      def get(self):
          pass      
  ```
  
  ``` python
  # api.py
  from flask_restplus import Api
  from feature import ns
  
  api = Api()
  api.register_namespace(ns)
  ```
  