pywrap: loaders: Add SaveXXX functions to submodule.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
demos: py_simple: Make use of loaders submodule.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: update python loaders docs.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>