Fix Missing `self.app`

When I try to migrate `flask-restful` to latest version, our project is down.
See traceback:
```
File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/usr/local/lib/python2.7/dist-packages/flask_restful/__init__.py", line 257, in error_router
    return self.handle_error(e)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python2.7/dist-packages/flask_restful/__init__.py", line 257, in error_router
    return self.handle_error(e)
  File "/usr/local/lib/python2.7/dist-packages/flask_restful/__init__.py", line 289, in handle_error
    help_on_404 = self.app.config.get("ERROR_404_HELP", True)
AttributeError: 'NoneType' object has no attribute 'config'
```
We init app through `init_app`:
```python
api = API(catch_all_404s=True)
api.init_app(app)
```
Method `init_app` does not assign app to `self.app`.