text: Title: Error handlers on namespaces causes error
  Body: Flask-restplus=0.9.0
  
  When error handlers are registered on a namespace and afterwards are transfered to api an error occurs. I got it working on my side by adding ".items()" on api.py:398:
  `for exception, handler in ns.error_handlers.items():`
  
  Due to the lack of time I cannot go through the contribution process myself.
  
  More info:
  For example when registering my error handler for a specific exception
  
  ```
  namespace = Namespace('endpoint')
  
  @namespace.errorhandler(ConflictExceptionClass)
  def handle_conflict_exception(error):
  ...
  ```
  
  When starting the program you will get:
  
  ``` ...
      api.add_namespace(ns_engine)
    File "/home/bevandeba/tools/python/local/lib/python2.7/site-packages/flask_restplus/api.py", line 398, in add_namespace
      for exception, handler in ns.error_handlers:
  TypeError: 'type' object is not iterable
  ```
  
  This issue is located at
  api.py:397-399:
  
  ```
          # Register error handlers
          for exception, handler in ns.error_handlers:
              self.error_handlers[exception] = handler
  ```
  
  Fixed by changing to:
  
  ```
          # Register error handlers
          for exception, handler in ns.error_handlers.items():
              self.error_handlers[exception] = handler
  ```
  