refine code

remove SwaggerApp.__root_url
replace SwaggerApp.__url2obj with SwaggerApp.__url2app
remove SwaggerApp._prepare_url, instead, all input url would be refined
by utils.normalize_url
jr_split

decompose json-reference into (normalized-url, json-pointer)
many refinement

- add SwaggerApp.__app_cache to union multiple SwaggerApp with url
- add SwaggerApp.__url_load_hook to patch url to load json.
- 'getter' parameter in SwaggerApp.create is removed, if needy, call
SwaggerApp.load instead.