Added a new method :meth:`.Engine.execution_options`

  Added a new method :meth:`.Engine.execution_options`
      to :class:`.Engine`.  This method works similarly to
      :class:`.Connection.execution_options` in that it creates
      a copy of the parent object which will refer to the new
      set of options.   The method can be used to build
      sharding schemes where each engine shares the same
      underlying pool of connections.   The method
      has been tested against the horizontal shard
      recipe in the ORM as well.