Xenonpy

**Bug fix**

* fix ``set_param`` method dose not set the children in ``BaseDescriptor`` and ``NGram``. ( 163, 159 )

**Enhance**

* Setting ``optimizer``, ``loss_func``, and etc. can be done in ``Trainer.load``. ( 158 )
* Improve docs. ( 155 )

**Bug fix**

* Fix `trainer.model = model` do not reset extensions error.