text: Title: Expose "Scalar" decorator at the root level of tartiflette
  Body: Currently, this is not straightforward to register its scalar types, the `Scalar` decorator is located at the `tartiflette.scalar` module.
  
  In term of API, it would be more consistent exposing this decorator at the same level as `Resolver`, `Subscription` and `Directive`.
  
  **Currently**
  ```python
  
  from tartiflette.scalar import Scalar
  
  @Scalar("DateTime")
  class ScalarDateTime:
      @staticmethod
      def coerce_output(value):
          return value.isoformat()
       
      @staticmethod
      def coerce_input(value):
          return iso8601.parse_date(value)
  
  ```
  
  **After**
  ```python
  from tartiflette import Scalar
  
  @Scalar("DateTime")
  class ScalarDateTime:
      @staticmethod
      def coerce_output(value):
          return value.isoformat()
       
      @staticmethod
      def coerce_input(value):
          return iso8601.parse_date(value)
  
  ```
  