Add resource decorator to Api class

Flask itself makes extensive use of the `@route` decorator for adding routes to the `app`. It would be helpful to be able to use this paradigm with `Resource` classes as well.

For example:
```python
app = Flask(__name__)
api = restful.Api(app)

@api.resource('/foo')
class Foo(Resource):
    def get(self):
        return "Hello, world!!"
```